Why do subs sound bloated or slow?

Bryon, I think you're on the right track but those two parameters you mentioned don't have to me mutually exclusive.

You can get the crossover point right either by ear or by measurement. Setting the time alignment (minimizing phase error) is probably more difficult, especially since most subs only offer absolute polarity or polarity inverted 180 degrees... the odds that either setting will be right by chance are near zero.

If the subs are not located in the same horizontal plane as the main speakers (and close to them) as is usually the case when they are placed in corners or close to the front wall, then unless you have subs with fully variable phase controls of at least 0 to 180 degrees, you'll need to do extensive trial and error to get the phase relationship correct between the subs and the sats. And yes, that's pretty much as hard as it sounds. The upside is that many subs offer full variable phase controls some even provide this feature via remote control so you can adjust it from the listening seat.

Frank

Under many circumstances, optimizing frequency response and optimizing transient response is a zero sum game.

Here’s what I mean: Much of the time and effort that goes into subwoofer setup is spent optimizing frequency response. That is typically done in one of two non-mutually-exclusive ways: Sub placement or digital EQ. Either of these approaches can result in a much flatter frequency response, making the bass sound less bloated. But both of these approaches can result in a sub-optimal transient response, due to the time misalignment between the sub and the mains. That makes the bass sound slow. To elaborate…

If placement is used to optimize the sub’s frequency response, then the sub often winds up closer to or farther from the listener than the mains, thereby misaligning the sub relative to the mains. Alternatively, if digital EQ is used to optimize the sub’s frequency response, then a processing latency is introduced into the sub’s the signal path, but not into the main’s signal path, thereby misaligning the sub relative to the mains. Hence either approach to optimizing frequency response can disturb the system's transient response, making the bass sound slow (or “out of sync”). On the other hand…

If you set out to optimize transient response, you will usually place the subs on a plane very close to that of the mains. Now the bass no longer sounds slow. But, due to room modes, this kind of placement often results in a highly uneven frequency response. Now the bass sounds bloated again...and so on.

The result of all this is that, when trying to optimize both frequency response and transient response, you often have the experience of chasing your tail. That is what I mean when I say that, under many circumstances, optimizing frequency response and optimizing transient response is a zero sum game. I do believe that there are solutions to this problem, e.g., the use of software that independently controls the EQ and the delay of both the sub and the mains. But most audiophiles who use subs don’t seem to avail themselves of that kind of solution, which can result in their systems sounding bloated or slow.

Bryon

Room placement and crossover frequency will also affect the sound.

I owned a REL Strata III and found it a very fast and tight sub. I have also heard subs that did not sound as calibrated...so it really depends on a number of things.

It is much cheaper to make a subwoofer that plays bloated and slow - so that is mostly what you find.

It is extremely expensive to make a subwoofer with low distortion, significant SPL output below 30 Hz and a good transient response (low group delay). It requires a very large sealed box with a very expensive high quality large woofer.