I agree with all that Bob has posted here, but I'd add that the single biggest
issue I have with augmentation is the difficulty in achieving smooth
integration. When "augmenting", you have a narrow range of
potential x-over frequencies that you're filling in "under". In this
case, let's say that means 30hz-40hz with your speakers in your room. If
there's lumpiness around or above those frequencies, I doubt that you'll ever
get good integration. My experience has been that the key to smooth
integration of subs is flat on-axis frequency response through the x-over
point.
That's why I tend to agree with Bob's comments about HT vs Music subs. A
really good sub designed primarily for HT will probably sound great if you
can get flat response through the x-over point, and even a so-so sub will
sound more than decent. I use decidedly so-so subs -by today's standards -
and, to my ear, they sound great because I've gotten them to integrate very
smoothly. OTOH, before I got them dialed in, they were a problem. My
system might sound even better if I used true high performance subs, but it
sounds awfully good right now.
Similarly, I suspect that even a great sub which is poorly integrated will drive
you nuts. IME, many rooms will cause too many peaks and nulls in full range
speakers to achieve flat enough response to avoid this problem. However,
even if you're fortunate enough to get the type of smooth bass response
from your main speakers that allow you to augment for flat response, there's
another issue.
Even in the absence of "lumpiness", there's the issue of level.
That is, even if your current speaker happens to be perfectly flat for 2 full
octaves above your chosen x-over point of, say, 35hz (possible, but highly
unlikely) how does that level compare with the rest of the frequency range
above? If the band from 35hz through 140hz (in this example) is perfectly
flat but elevated by 5db or 7db relative to the mids/highs (actually pretty
good results relative to many rooms I've had), you'll get good integration, but
probably too much bass overall.
If you can augment successfully, it may be a better choice, because you can
get the active x-over out of the chain. However, IMHO, replacement will
usually be a better choice because it gives you broader freedom to choose a
x-over point that is both smooth and appropriate in level. IME, though, it's
hard enough to get it right even WITH the flexibility offered by an active x-
over. ( That's why I also use EQ in the bass. )
Good Luck
Marty