Nad c370 sub hook up

[Burchill]

So the setup nad c370, klipsch rf82's and a Velodyne cut 15 (which is the issue) So my nad c370 and the speakers with out sub hooked up work fine, however when i add the sub in the mix speakers still work great but the sub is boomy (i know but its a sub)lol well I have to turn the volume all the way up on the sub as well as pretty much half way on my nad to get any base going, what the issue I have the sub hooked up via speaker levels speaker wire s into the sub then speaker wires out of sub to speakers, of course the speakers work fine when hooked up this way buy the sub should be making my ears bleed at these levels and this is a second hand sub i bought last night and it did seem to work when first hooked it up been then i changed my wires to longer ones 5 ft all same length then the base seem really low almost muffled sounding.