Sounds like there could be a problem in the sub if you have not changed any settings just before this started to happen.
Unfortunately you don't give us any details on your other speakers or sub placement in the room (corner placement gives the sub the highest volume) I couldn't get a picture of the back of your sub because the M&K site is down. They have gone bankrupt recently. Not sure how the left side RCA plug is labelled, but it sounds like you are using the correct input.
I would start by making sure you have the sub dialed in correctly. Many people overdrive their subs with a volume setting that is too high. I would check the following:
Be sure you set up the speakers and sub using a Radio Shack analog (not digital) volume sound pressure meter. They are about $40 now and much more accurate than your ears (this really makes an audible difference in the surround level smoothness). A DVD like the Sound & Vision Home Theater Tune-up by Ovation Software is also a great tool (avialble online at amazon, etc).
You should be using the crossover and volume settings on the receiver to adjust the sub. Set to 80Hz the receiver sub crossover setting instead of 100Hz unless you have very small speakers. The receiver's speaker setting should be set to "small" for all speakers unless your left and right are very large speakers (the small setting might still be best with large speakers). The center and surrounds should always be set to small unless they are huge like the mains. Use the bypass setting on the sub like you indicate.
Turn any bass or EQ controls on the receiver to zero or less (no bass boost).
On the sub turn the crossover setting to the maximum setting (the crossover is set by the receiver and the sub should be set to max to avoid "cascading" filters) and the volume setting to 10 o'clock to start.
Set the speaker volume settings on the receiver for the sub (and all other speakers) using the receiver's test tones (or set-up DVD) and the Radio Shack meter. I'm guessing that you might have the combination of sub plus receiver volume set too high for the sub.
Make sure you have a good tight connection both at the sub and receiver with you subwoofer cable.
My other speakers are a M&K S85-C center, Infinity right and left, and Boston Acoustics in the rear. All but the center are absolute crap. I have them all set to small. I'm actually looking into a total HT upgrade and I'm trying to figure out if I need to budget for a new sub as well.
I set the receiver to 80hz, base is a 0. I then cranked the sub Low Pass Filter as high as it would go to 125. I'm guessing this is the crossover you suggested I set high? Sub volume is set at 0db currently, moved it up from -3db. And it doesn't seem to be popping. I put in lord of the rings and shook the entire house so I'm going to assume the Low pass filter was the issue unless you have other ideas.
It still goes crazy whenever I turn the sub volume up but I think that's a different issue.
Yes, I was talking about the crossover/filter knob on the sub. This way the crossover is handled by the receiver and not duplicated on the sub, called cascading filters.
When you were originally talking about adjusting the bass level in your first post I presume you were talking about the volume knob on the sub. You should set this and never touch it again (unless you have to adjust it in your intial speaker set-up routine). From then on, only use the sub volume setting on the receiver. It is much more accurate than the knob control on the sub and you can remember settings you have for different sources. For example, you might determine that you like 0 (zero) for HT, -1.5 for rock and -2.0 for classical music. Again these initial settings should be set using a RS meter if at all possible. You will find that by ear many people will be as much as 2 decibles off from having all 5 speakers and the sub at the same volume output level. With the RS meter you will be able to get the settings within 0.5 decibles of each other for all the channels.
I think you have the sub volume control set too high. It would be nice to know what the actual knob setting are in decibels, how low below zero and how high above zero. When you now turn the sub volume control up past your new zero setting you might be overloading the amp and hence what you call going crazy. If it is equal on both sides for min and max then set it approx at 10 o'clock. Otherwise set it to below the zero mark. Maybe -3 to -6. This is simply a reference point and then make all further sub volume adjustments on the receiver. If the sound of the sub is too loud or too soft in comparison to the other speakers then adjust the sub volume setting in the receiver only. Then use the master volume control on the receiver for listening to movies or music. After you get everything set to your liking, then you should not be turning up and down the volume knob on the sub or the sub volume setting on the receiver during playback but only when changing sources like from HT to music if you find that you like more or less sub output in those cases. I hope this is somewhat clear.
You have a mismatched system there with three different brands of speakers. The sub can be from any brand but the other 5 speakers should be from the same brand and the same series so that they have at least the same tweeter and if possible the same midrange (called the same "voicing"). (others here will say the surrounds can be from any brand but I strongly disagree). The same voicing all around lends to much smoother pans across the front and ultra smooth and enveloping surround music and movie surround sound. It makes a very audible difference.
I cannot find the specs on your M&K sub. Woopps, here is some info:
Built-in 150 watt RMS amplifier
Active Headroom Maximizer IV design to prevent clipping
Adjustable 50Hz-125Hz low-pass filter with bypass
Frequency Response: 20 - 125 Hz ± 3dB
Weight: 64 lbs
Dimensions: 23" H x 15.25" W x 19 5/8" D
Warranty: 10 years, 5 years on the internal amplifier
This is one fine sub. I would definitely hang on to it. MK is known for very fine and well built subs and speakers. I would start upgrading your system with the 5 speakers first. You could get 4 more S85's or other M&K's with the same exact tweeter. From the owner reviews on audioreview.com the S85 seems to be a great speaker. Five of them and your M&K sub would make a very fine system. The crossover point at 80Hz might be OK or possibly 100z on your Denon. I don't have the exact specs on the S85.
Thanks for the lesson!
I haven't been using the receiver to adjust settings, but that makes total sense. All my speakers were purchased 10 years ago when I was working in a video/tv store because I got everything at cost - they are all a mismatch since I just bought things when I needed them. And since it was at cost I just got whatever my coworkers suggested were good - none of them at the time talked about matching! Only recently have I really gotten into learning about HT and discovered a lot I didn't know.
My bass level options are: -6, -3, 0, 3, 6, 9. The 10 oclock position would basically set it at -6, I'll give that a try and see what happens.
My S85 has been dropped a few times (college drunks back in the day) and it rattles so I'm thinking that's got issues as well even though it sounds ok. I'm planing to look into B&W 600s, Paradigm and some of the other speakers often talked about for all 5 speakers to get the matching set.
I'm planning to do speaker upgrade, then buying the projector and the blue-ray player in a few months. I've decided to hold off on the HD TV upgrade since I love movies most. Plus that HD Tivo has to come down in cost one day. Only challenge is we'll be renting houses for a while and the HT will also be our living room until one day we get our own house and I get a media room.
The idea with the 10 o'clock setting is that if the control knob is symetrical then 10 o'clock equals about 1/3 of the gain on the volume control. That is a good place to start. From what you list, -3 probably is a good place to start.
B&W and Paradigm are both excellent speaker brands. Velodyne makes great subs.
I'm into best bang for the buck and look closely at the direct to buyer online only brands. For both speakers and subs check out HSU, SVS, Outlaw Audio, Emotiva, AV123, Ascend, Aperion and Axiom Audio. I own Axiom speakers and an Outlaw sub and am extremely happy with the sound and especially the great prices. SVS and HSU also make super subs. Axiom's surround speakers are superb and the new Emotiva speakers got a great review recently. All these sites list reviews on their products. Good luck on choosing your new system.