Recently, I switched my Phoenix Gold Xenon 1200.1 amplifier, (was going to a 13W7 @ 3 ohms), to power my six 12" subs (final impedance 1.33 ohms), and put my HiFonics Brutus 1605D amp that was powering the 12's, to my W7 (but dropped impedance to .7ish ohms instead of 3).
I couldnt be happier. The Hifonics works very well at that level, hasnt overheated once, and I'm getting more power from it this way, too. I'm going to have it metered, but I'd bet I gained probably 1.5-2 dB legal on a termlab by doing this.. It's pretty crazy, I was surprised by the results, honestly.