Is a subwoofer 'overkill?'

Sounds like it's not set up properly. Possible that:

1) the crossover point is set too high so that it is doubling up frequencies already output at good volumes from your main speakers
2) the volume is still set too loud
3) the placement is bad

I recently added two subs and found that an SPL meter helped a fair bit to verify what I was hearing. I don't have true flat response but I have been able to get it close enough that my system definitely sounds richer without the bass playing any louder than the rest of the music.

Bottom line - keep experimenting - this takes days, not hours...

Pull the sub further away from the room boundaries to tame the loudness factor, or sell the wife... :^(

Sounds to me like there isn't optimal integration. When crossovers, phase & level are correctly adjusted, there should be no distinct impression of bass heaviness or low frequency accentuation, just a sense of greater acoustic space, a clearer acoustic window into the music that's fleshed out with harmonic completeness. A good start for inexpensive help is Rives Audio's calibrated test cd that's designed to work with the Radio Shack SPL meter. It really helps to use a bit of calibration dialing these things in. Level, contour, phase controls, etc. don't always have effects that meet logical expectations.

Assuming you're running a powered sub, connected with RCA's.

I'd suggest using Rothwell 10db attenuators, which will allow you adjust the pot on the sub to a much more optimal setting.

I use this method with my Sunfire sub and it allows for a MUCH smoother intergration with my main speakers.

It really makes a HUGE difference.