Why do subs sound bloated or slow?

"Many subs have group delays of up to or more than 1 cycle. Of course this is audible."

I don't remember the exact group delay figures used in the study I read, but they were consistent with what one would expect from a well-designed vented system. Using digital signal processing, group delay was isolated from frequency response (don't ask me how they did it). On test signals group delay was slightly audible, but on music it was statistically inaudible. The audible effects normally attributed to group delay were replicated by a frequency response curve mimicing the high group-delay system, even if the group delay is eliminated by DSP. So while large group delay correlates very well with perceived "slowness" in the bass region, it is not the actual cause.

I currently have my subs next to my mains, slightly forward. They are sensitive to movements of one inch. I can play a passage and it will sound right, but if I move the subs an inch forward or back, and it's out, and stays out if I move it farther. In previous setups, I've been able to introduce a delay to the mains relative to the subs (in Pure Music, for example), and I had the same results: the coherence of the sound was sensitive to delays that were a fraction of a millisecond.

So time alignment is crucial, and I don't think the subs will ever be truly integrated in a system until that is achieved. I think once it is tackled, EQ is available to deal with the frequency response of the room. That's my current approach, and I don't think I've sacrificed much, except that there are subwoofers sitting in not-inconspicuous locations in my living room.

Crossing over to full range speakers....at too high a frequency, is guaranteed to give you that bloated feeling.

Certainly group delay and frequency response are intertwined.

I suspect that "masking" is what makes the effects audible. What I mean is that you can't perceive accurately what is happening to the bass response except by what it "masks" in the higher frequencies.

If a 20 Hz signal suffers a group delay of say 30 msecs - what happens to the harmonic distortion associated with that sound...are they also delayed by 30 msecs?

If the harmonic distortion is significant and delayed by 30 msecs then you may have a very audible 80 Hz harmonic that is audibly delayed by 30 feet. I cannot help but suspect that this harmonic will make the bass sound "slow", "bloated" or "muddy" by masking higher frequencies.

In essence, just consider the entire harmonic distortion from low frequencies (below 50 Hz) all delayed by the group delay - this smears out the bass response and masks transients to the ear.

I agree that a hump in the frequency response will ALSO cause masking by emphasizing certain frequencies by as much as 5 dB more than they should be and making the balance of sound such that you do not properly hear the higher frequencies (sound of the high frequency hit from the stick hitting the drum head).

Sound Engineers know that they can change the "speed" or attack of bass notes by altering the higher frequencies - emphasis is added for greater attack and de-emphasis to make the sound more laid back - in my opinion they are playing with teh amount of bass "masking" when they do this

You shouldn't really "hear" the sub. If you do, they are too LOUD!!!