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