Magkids, what you're describing sounds like a lack of pitch definition to me. It's likely either caused by the 390 overpowering your mains or the 390 is exciting a room mode that your other sub could not. More output, either via a bigger amp or porting for more low end output, will not help this type of situation.
A room mode is a frequency, that do to the physical dimensions of the room, is overemphasized. You could also end up with a null, which is no or severely reduced level at certain frequencies. Running a sweep with REW will help you identify any problem frequencies. Room treatments and experiementing with sub and/or listening position can help compensate to some extent. Depending on which version of Audyssey you have, it may be able to improve the response a little bit...you can't really boost out of a null, but you can reduce a peak a bit.
The detail you're looking for in explosions really comes from the higher frequencies played by your mains. If you listen to content with just the sub, it's more of a thud-thud. For a simplistic example. The lowest note on a standard 4 string electric bass is 40ish Hz. It's the harmonics of an instrument that give the instrument its audio signature. So, if you play the lowest note on that bass, the first harmonic will be at 80ish Hz and reproduced by both the sub and mains about equally with an 80Hz crossover point. Obviously, the higher you go on that bass, the more of its signature comes from your mains while the fundamental frequency is played by the sub.
Did you run the Audyssey auto calibration routine or are you doing level calibration manually with an SPL meter? The auto routine usually does a good job of getting the levels matched, but room acoustics can sometimes throw them off. If doing it manually, you can really pick any arbitrary SPL to level all channels to, but it's commonly accepted to calibrate to 75/85dB depending on whether your test tones are recorded at -30/-20dBFS. The Onkyo x05 series, and likely other generations, internal tones are -30dBFS so 75dB would be the target. What receiver/version of Audyssey do you have? Some versions actually apply EQ to the sub channel, while some only adjust the level/distance.
I recommended turning up the amp because Behringer doesn't list a range for the gain structure as would be expected for an amp with variable gain. That suggests to me the "gain" knobs are really just attenuators. By running the A500 at full gain, you reduce the chance of overdriving one of the low level upstream preamp stages. FWIW, my sub channel ends up at something like -3/-4 and my sub amp has about the same gain (29dB) as the A500 is rated at (28dB). Even then, I'm running a little bit hotter than the auto routine suggested...room is 24x16x9 with the sub about 14' from the main listening position.
-Brent
