Ramy: as per your request, here are some basic suggestions. Obviously, some Krell's make for great bass amps but there are also some others that are very worthy contenders. Before mentioning some models though, here's a thought for you. Quite honestly, it is what i'm doing with my main system.
Like you, i have four 12's per side, each of them rated at 4 ohms. In order to maintain dynamic headroom and keep from going too low on the impedance, i split the four drivers per side between two channels of an amp. That is, i run both channels of a stereo amp for one side. I have two 12's on Channel A ( 2 ohms ) and two 12's on Channel B ( 2 ohms ) of the same amp for the right channel. An amp with a small output inductor just would not work very well at this low of an impedance. I then have a duplicate amp wired up the same way to drive the left channel. This is more expensive, but works killer to say the least.
I've used the same approach for the mids and the tweeters too. In effect, i have six stereo amps acting as mono-blocks in an actively tri-amped system. All twelve channels are driving multiple drivers per frequency range. This allows them to share the low impedance load without any one channel or amp ever getting stressed. Things are "cleaned up" even further by actively crossing the amps with a sharp slope, therefore limiting the bandwidth that each amp has to cover. You could do much the same thing with your woofers should you choose to go that route.
As for the amps, i'm running two Perreaux PMF-3150B's with two ohm loads per channel. This results in about 800 wpc x 4 channels driving my subs. If i could find them at the right price, i'd like to snag two PMF-5150's. I "think" that the 5550's would also work well here too, but i'm not sure if these use iron core transformers or toroidals. I don't know when Perreaux switched to using toroidals, but i think it was after the 5550 series. Having mentioned Perreaux's, i wouldn't use 2150's, which are relatively abundant and cheap, for low frequency reproduction. Even though these were rated at 200 wpc @ 8 and clipped at about 280 per channel, they were earlier designs. Peter Perreaux revised the design of that circuit and the results are FAR superior in terms of bottom end "oomph" with later models.
Other amps worth considering for "booty busting bottom end" might be Classe' M800 or M1000 mono-blocks ( 800 wpc rms and 1000 wpc rms mono-blocks ). These are huge behemoths to say the least. Other BIG Classe' amps ( 300's and 400's ) might be okay for bottom end, but i don't really like them at higher frequencies. I would not doubt that these would be better choices than the Bryston that you currently have though. As many folks here know, i am not a fan of Bryston amps even though their customer support is the best in the business. I know that Pbb won't be mad at me though as i just recommended another Canadian company to take their place : )
As mentioned above, using a pair of smaller amps might also do the job. In this case, you might try a pair of Aragon 8008's or Electron Kinetic Eagle 7's. A factory MODIFIED ( not stock ) Sunfire ( or two ) or ( preferably ) a Sunfire Signature amp ( or two ) would also work well. Any / all of these amps could be further updated / optimized / modified for very specific "low frequency reproduction". Some of the earlier Threshold amps might work well, but they would require a lot of upgrades compared to the others mentioned. I've never considered any of Nelson's designs that i've heard to be "killer" in terms of "bass thump" although i love everything above that range on many of his amps. The newer X series might be an exception, as these amps have absolutely huge power supplies. I would not even think about using one of his Aleph's or the newer XA series for something like this though.
Hope this helps and gives you some ideas to think about. I know that using two stereo amps with all channels driven is not something that most folks would normally do, but it definitely has its' advantages in this type of situation. Sean
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