Ruining music with a cheap sub

There are huge differences between bad/cheap subs and good/expensive subs. It's called distortion. And no, low bass is not *just* felt. Besides, a sub that's reproducing the bottom octave is rolling off in the second (and maybe into the third) octave, so it's going to affect your perception of a lot more than just that bottom octave.

I don't know of any good $100 subs, but there are some good $500 subs out there. And I'd say the point of diminishing returns hits well below $5000. YMMV.

"Sloppy loose bass from a cheap sub is worse than no sub at all IMO."
Chadnliz

In my opinion too.

With respect to subwoofer directionality, there's a wonderfully funny thread where some of the participants get into a SPIRITED debate about that very topic. It also seems to have some pretty good info. Here's the link:

Stereo versus monaural subwoofers debated right here on Audiogon

03-11-06: Mdhoover

"Sloppy loose bass from a cheap sub is worse than no sub at all IMO."
Chadnliz
In my opinion too.

With respect to subwoofer directionality, there's a wonderfully funny thread where some of the participants get into a SPIRITED debate about that very topic. It also seems to have some pretty good info. Here's the link:

Stereo versus monaural subwoofers debated right here on Audiogon

Md, thanks for the link.

I totally agree and believe me, I know how a bad woofer sounds like. Perhaps I used the wrong word, maybe decent but relatively low cost(below $200 used?) would be the right phrase. Isn't it true that distortion is proportional to volume? If I am not looking for something to knock the stucco off my house, and as long as I watch the distortion threshold, then I shouldn't have to go break the bank to get a highend sub. I just don't quite understand why someone would spend thousands on a sub. Wouldn't it make more sense to spend that extra money to get better speakers?

I remember someone once told me that two subs are no better, if not worse, than a single sub(in a stereo setup) because one might cancel the other one out. Is that true?

I used a Def Tech 15" subwoofer that sounded great on movies, but just didn't sound right when listening to music. Although the bass was powerful, it seemed like it was just not keeping up with the music and kind of gave me a headache. I listen at moderate volume.

After eventually upgrading to an M&K 350 sub, I realized that a good subwoofer makes a huge difference. No more headaches.

Kilsho,
Although I do not claim to be an expert, I'll try to give my less than authoritative answers to your questions, based on my intuition and limited experience.
1) In terms of cancellation, I believe that's an issue if the subs aren't set up meticulously, and that it CAN happen.
2) I'll never say never, but I suspect that most very low priced subwoofers don't sound exceptional at any volume, although lower volumes would be expected to mitigate distortion.
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But there are other important things, besides lower distortion, that tend to correlate with increasing quality such as:

->How well will the unit integrate with your system?

The older Rels (and maybe the newer ones, I just don't know so I can't say, that's all) were designed to integrate seamlessly with any or almost any system. Audiogon member Quadophile has written an EXCELLENT review of the Rel Strata Mark III on Audiogon:
Rel Strata Mark III by Quadophile

->Not just how loud, but how DEEP can it go?
The Rels are (mostly) designed for REALLY low frequencies, and the crossover is generally set QUITE low. Mine is a Rel Storm 3 set at 27 hertz. Quadophile has his Rel Strata 3 crossover set at 23 hertz. The idea with these is to overlap the rolloff curve of the mains, so that the upper rolloff curve of the sub be added to the lower rolloff curve of the mains, yielding an almost FLAT curve (when summed) down to below 20 hertz.

Lower priced units are generally not designed with such a t approach in mind. In fact, they tend to sound boomy, meaning that they're overlapping the low end output of the main speakers. The cheapest of the cheap don't even control cabinet resonance very well. People unfamiliar with what good bass is SUPPOSED to sound like (I'm not talking about you here, by the way) think that's great, but they're really hearing augmentation of stuff that's probably more like up in the 50 to 80+ hertz range.
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3) The topic of satellites or monitors (or, sometimes even so-called full range speakers) for the main speakers paired with one or more subs has also been discussed, with the comparison being between dedicated, full range mains. Folks differ on their opinions about this, and a very good thread about that is this one:
Sub/sat versus dedicated mains