Benchmark or McIntosh....

Hah - older Krell gear for sure.  The new Krell Amps don’t run hot at all.  For what I think you are looking for out of the amp, Krell and Max’s are the 2 American Made brands that really built their reputations on being able to drive almost any speakers, even 4 ohm or speakers that dip into the sub 4 ohm realm.  They went at if in different ways, Mac with their proprietary process that promises to deliverer stable power across the spectrum, automatically matching the need of the speakers, Krell designs their Amps to double down on power as Ohms drop.  By no means are these the only 2 that offer Amps are designed to drive difficult loads, they are the 2 most well known for it.  Hegel would be another great option.  Music Fidelity would be another along with Parasound.  If it’s drive and power, as others mentioned, the new Clas D amps deliver that in spades, run cool, more compact footprint and efficient.  No experience with the new Class D and not everyone falls in love with them, seems to be you either really love their sound or prefer Tube or old school SS Class A or AB amps. 

shoup1cobra,

Your room size is your limiting factor. It seems you don't even have enough headroom to begin with. There is simply not enough " air volume" for 21 ft. of subs to breath properly in 7' ceiling room, never mind your eardrums. Your own skin maybe the only thing saving your nirvana.

@roxy54, @jasnobourne71, @ghasley  have all nailed it for you.

Best regards, Chris.

I love Tube sound I.have 4 Tube amps EL 34....KT 88 and 845’s...and Decware Zen Triode.......The new Aavik Amps ( Denmark).....have the warmth of tubes and the detail of SS with an EXPANSIVE soundstage. The Best of all worlds....High End Class D is the best sounding Musically of Any format at any $ IMHO. AGD is also a great amp....Peachtree ganfet 400 is a very musical class D amp for a Budget minded person w/ a tube pre.

Every amp I've had has been SS. Several Adcoms, Sunfires, Musical Fidelity, and the Wadia...which someone just bought, so I'll definitely have to do something and soon. 

@onhwy61 Yes, exactly. Pushing boundaries and the limits of this system. The 21s are surprisingly musical...they come from the sound reinforcement/live sound realm. I would say more musical than the 18's they replaced and the 15s before them. 

@jastralfu It's my third set of NHTs, I 've owned since new back in the early 2000's. Stereophile Class A limited back in their heyday. Only work that I had to do was replace the crossovers a year ago (I think room correction "adjusted" a little too much and they went out)

SEVEN 21-inch subs? OK, here’s my tenuously related old-audiophile anecdote.

When I was a kid in the early 70s, some friends dragged me to an attic bedroom in an old hippie house to smoke weed with two older Haitian guys that I didn’t know. The room was something like 25x6 feet, long & narrow.

At either end they had one of those giant metal horn speakers you see mounted on towers at rock festivals. Almost as tall as me. I should have run as soon as I saw them.

These guys were really proud of their system, but were also very scary. Without saying a word, one of them cued up Mountain’s first album. Mountain.  Of course.

The lead-in groove surface noise was deafening, but when the music started, I couldn’t hear anything at all. The only thing I remember is an intense sheet of pain bisecting my head. No sensation of hearing sound.

And to complete the mind-f**, the second guy put on a strange display while the music was playing, furiously dancing around the room waving a big-a** knife under our noses & grasping a humanoid cloth doll, as if to psych us out with some sort of drugged-up faux voodoo ritual. I’m sure they thought it was funny.

We left as soon as we recovered enough to rub two thoughts together. Even today, decades later, I still can’t hear "Mississippi Queen" without flashbacks to that terrifying experience. The wages of sin.

And that’s why I became an audiophile!