"...How do you know the equalizer is needed to begin with, if not by using your ears..."

When speaker placement and environnmental options have been maxed-out, it's time.

My preamp/control center has four tone controls w/center freqs @ 50Hz, 100Hz, 10kHz and 20kHz. My main concern was in the bass region and neither of the low freq controls were selective enough to ameliorate my particular problem. "Hearing "something" is only the first step...and as MM alluded to, unless you live in an anechoic chamber, generally it couldn't hurt.

"...how do you know you adjusted the equalizer right, if not by using yours ears?..."

I purchased an SAE 2700B stereo half-octave EQ...avoid non-independent "ganged" units; there will most likely be differences between channels. I felt the half-octave unit was satisfactory for my purposes given the alternative of buying two, single channel Altec Lansing one third-octave ones.

This is where the minimal tools are required...I used a Crown test record(I'm into analog) and an SPL meter. I had access to a pro unit and also purchased an RS unit. Both gave reasonably coincident results and the RS meter was more user(wallet) friendly.

On a legal pad I wrote down the frequency bands as listed on the album cover. Unfortunately, the disk was divided into one-third octave bands, but more on that later. After setting the reference SPL @ 1kHz pink noise, I simply began to measure levels at each frequency snippet for each channel and note them on my pad. I then transferred the list to my graph paper(you could get fancy w/ log paper, but regular stuff is sufficient IMO). I did this because of the diffs w/ 1/3 vs. 1/2 octaves. As a result, some compromises were made and the visual aid of the graphic plot helped resolve them. I then adjusted the EQ pots to the inverse of my graph. it was done three times to increase resolution. The vast majority of sliders are in the "cut" mode and the most activity is in the region below 120Hz. As PCT said, his subs are where the EQ is doin' it's thing.

Now here's where we get to my previous mention of EQing vs. gain. If done correctly, each band is at reference level so the overall volume should also be at this level; switching the EQ in and out of the signal path should result in no apparent volume shift, only a re-shaping of the frequency response. Anything else is wrong IMHO and something was probably done incorrectly.

After listening, I decided "flat wasn't where it was at" for me and applied a gentle roll-off above 10K as it sounded more "natural"...I have no intention of getting into a debate about absolute ruler flat response and my initial results which were, generally speaking, +/- 3db...in practice the ear isn't all that acute to notice IMO...smooth linear bass extension and natural sounding mids and highs is more than sufficient thank you.

When all is said and done, I am of the opinion that attempting to use ears alone is open to all sorts of vagaries...different recordings just plain sound different and trying to accomplish this task with them will result in less than "correct" results, at least initially. If you choose to tweak things to your preference as I did, it should only be after you have established a baseline reference point. BTW, I still rely on tone controls to adjust for varying qualities in source material...

jimHJJ(...and that's my story...)