does a subwoofer kill stereo sound?

In response to the original poster, a subwoofer has a pretty simple job to do...play frequencies in the 100 to 20Hz (or lower) range.

It's apparent you want more bass. Provided your bookshelves are capable of playing down to 80Hz or so, and you cross them over properly, there's no reason why you wouldn't enjoy the added low end reinforcement a subwoofer provides, with one caveat...some sort of EQ would provide the best integration with your room. The SVS AS-EQ1 is an amazing little piece of equipment.

So your decision really depends on your budget. A decent sub and EQ could cost from $1500-$2000. Are there full range speakers out there in this price point that interest you? If so, would they sound any better than your admittedly "great" bookshelves?

The added benefit of a sub (or my preference, dual subs), is that you can just turn them off, giving you added flexibility. Or just turn around and sell it(them) if it's not to your liking. You still get to keep the speakers you like.

There is no need to spend lots of money on a "state of the art" amp to drive a "state of the art" sub.

Disagree. Most subs (I mean 99%) simply distort so badly that they ruin the sound (at least at usable SPL's in the ultra LF range of 20 to 30 Hz).

It is EXTREMELY difficult to produce low frequencies at reasonable SPL without distortion and therefore extremely expensive to do it right (to audiophile standards).

The same can be said for speakers - it is just plain difficult to do ultra-LF well - the manufacturer may claim performance to 20 HZ on a full range speaker but what they don't tell you is that you will be hearing upwards of 10% THD at anything nearing useful SPL levels.

Distinct bass lines from drums is something that can be heard on a good system as the timbre is quite different (even if they may overlap in frequency range). The trick is to have a critically damped design as opposed to a ported resonant design. (although resonance gains efficiency and SPL it has a down side in that it destroys timbre and will make differences between bass and drums that much harder to hear)

Here is a little more why I believe resonance and THD is important in ultra LF.

Resonance is important because of "masking". Masking is what is used to create lossy MPEG files that sound almost identical to lossless files. Masking is the well known fact that loud low frequency sounds will mask higher frequency sounds to the human ear/brain system. Therefore, if you cannot hear these higher frequencies (due to LF sounds) then this detail can simply be removed from the audio files (reducing the file size dramatically), whenever there is "masking" going on. All you need is a clever algorithm to recognize when masking will occur and you can compress any file.

THD is especially important in ultra LF because your hearing sensitivity increases dramatically from 20 Hz to 100 HZ. Typically you are some 40 db more sensitive in hearing at 100 Hz compared to 20 HZ. This means that even minuscule levels of distortion in the ultra LF (20 to 30 Hz) can still amount to highly audible harmonic distortion effects at 100 Hz and above (altering dramatically the timbre of the sound). Since the distortion is harmonic it generally does not sound bad (and will not change the fundamental note) but most subs, which distort badly, will make everything sound extremely bass heavy (making it much harder to hear details) Of course, since it doesn't sound bad most people think these subwoofers are amazingly "impressive" - as they hear such heavy handed bass (deep bass they think). Actually a good precision subwoofer should not sound "impressive" at all!

I've tried at least 4 different amps ( 3 were bridged Stereo amps ) on my passive 18" sub , the better and pricier amps were very important and not just for impact , For 2 channel music .
I also tried some different crossovers , including one with room correction , hate to say it , but the more I spent the better it got .
As good as it is , We seem to listen to most music without it , but its a must for watching movies .

Put the sub between the two speakers. Problems solved.