what is the best sub amp.

I hate to admit it (being a so-called audiophile), but the best sub amp I ever used was a QSC MX3000a. 1500 W/ch into 2 ohms. I tried a Krell KSA-150, Crown DC300A, Bryston 4B-NRB, but the QSC took complete control. At about $1000 new it was also a bargain. Find it at your local musician's supply.

i also tried the bryston. i liked it but thought the krell was better. the qsc is a beast.the crown dc 300 is to weak. the macro i have now is 760 in to 8 1150 into 4 and 2500 mono into 4 ohms. i might stay with the crown but i also thought maybe a big boy krell would be nice. thanks guys keep the ideas rolling.

Eagle amps! Kind of difficult to locate one though because most of their owners do not want to part with them,
very high current design with taut bass.

A friend of mine who has had a LOT of different amps and a LOT of different multi-driver subwoofer arrays thinks that the Eagle's produce the best bass out of all that he's had. Maybe not the most powerful in terms of sheer grunt, but the tightest and most tuneful. He also likes the bigger Aragon's, at least for bottom end.

Nothing i've had or heard can touch the big Perreaux's for bottom end oomph and slam. I will stress that this comment applies to some of their models more than others. I've run BIG, BIG amps rated for more than twice their power at the same impedance and the Perreaux's still win that specific category. If you are strictly looking for bottom end thunder, the big Perreaux's ( models rated for at least 300+ wpc @ 8 ) will deliver.

As to the Bryston's, my experience with them is with their older models. I had a 4B ( 250 wpc @ 8 / 400 wpc @ 4 ) and i was not impressed. It did not do well into low impedances and i was constantly driving the amp into clipping. The amp was running quite well, as it was completely gone through by Dennis at Bryston USA. Phenomenal people to work with that truly do stand behind their products.

One more thing to add about amps and bass response. Amps that make use of "old school" iron core transformers will typically produce better quality bass than an identical amp using a toroidal transformer. Toroidals are simply cheaper to produce and less expensive to ship ( less weight ). Besides that, they tend to look "hi-tech". From talking to more than a few, most "honest" engineers will support this point of view for more than a few technical reasons. Sean
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Try the Bryston 7b-st/14b-st before you give up on Bryston, they are a lot bigger than the 4b-st with gobs of grunt. Krell is a good choice as a sub amp as well (provided that's all you're planning to use it for...), though kinda pricey for "just" a sub amp. And as you've already figured out, you're gonna need a lot bigger Krell than a KST100 to get there from here. In the end, you may go through a lot of amps and samoleons and still not do any better than that Crown. If it were me, I'd keep it and spend the dough elsewhere.