Quad 909 might be nice
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After 20 years of Quad experimentation, the Esoteric Audio Research 509 gets my vote. Effortless and neutral when much higher powered amps suffered. The capacitive load probably won't work with any Class D amps because of the feedback scheme. Both aforementioned amps have tertiary feedback windings in the outputs. Those windings prevent the feedback from getting out of phase due to the capacitive nature of ESLs -if I understand it. Otherwise, the best Class A sand amp you care to buy. I have an old Bedini 100/100 workhorse from the eighties that I keep around just because of this. It is not as transparent as some of the modern amps, but handles the load with aplomb and works in a pinch. BTW, the Bedini 25/25 was hot with the 57s back then...
I tried Nuforce 9.02's with my Quad 63 USA's just prior to selling the Quads. It was not a good match in my system either. Made the Quads almost dull and lifeless...I have used various tube amps with the Quads.from low watt El-34 and 845 SET to 60W Kt-88 amps..all with excellent results..the best amp I used on my Quads was an older Threshold S/200..just a perfect synergy of balance, warmth, and transparency with enough power to grip the panels. I also used an older Class A Bedini BA-801 that worked great too.
Im not sure if Class D amps are the problem. Another AgoN memeber who runs Quad 989's runs a pair of Channel Island Audio D-200 monoblocks and they seem to do just fine..Maybe its the Nuforce amps in particular..dont know. But they did not work well on my Quads.
I have them on my Emerald Physics speakers now with exceptional sound.
I had Quad 988's and tried a number of amps, inluding: Quicksilver Triodes (50W 6C33C), Cary SLM-100, Cary SLM-70sig, Wolcott S-220, EAR 509 mkii (509 tube), Odysey Stratos Dual Mono, Aragon 2004. Unless otherwise specified, these amps generally use EL-34 tubes, which I think are a good match for the Quads. Contrary to 4EST's experience, I was pretty dissappointed with the EAR - to me it lacked a certain amount of delicacy and fine detail resolution. The SLM-100 was also a bit of a dud (it uses KT88 tubes), whereas the SLM-70s sounded very fine, with enough ooomph and very good fine resolution. The Wolcott also sounded good, maybe slightly better than the SLM-70, but not enough to justify the cost. The Quicksilver sounded a bit aggressive I thought - maybe due to the 6C33C tube. The Stratos (solid state) was acceptable - no show stopper. A big surprise was the lowly Aragon, can be had for 400 dollars, and it had great synergy with the Quads. Although it had the slightest bit of grain, the dynamicism, soundstaging and imaging it achieved were really something! I would recommend picking up one of these as a "backup" and use it while you audition others and in case a tube amp breaks. I think some people might prefer it. I would imagine that the Aleph 3 would sound good, but maybe a bit polite (thoughts, Mozartrules?). I would suggest getting an EL34 based tube amp, or maybe one that can switch tubes. What is your budget?
I just noticed your thread is for the 2905 rather than the 2805. Since I had the 988, you may want to have an amp with slightly more power than I mentioned, I don't know.
Another huge improvement I got with my Quads was a preamp that was very dynamic. The CAT Ultimate was certainly that, and really brought the Quads alive. If I had the money I would consider CAT gear (though I didn't thoroughly enjoy dealing with the manufacturer). I'm sure it would take the Quads by the horns and make them jump through hoops!
Newer Quads have a lower impedance than the older ones, as Quad contiues down the same path that Martin-Logan took, trying to get transistors to work with ESLs. But the nature of ESLs is that tubes will drive them without high end empahsis and better bass impact. So to deal with the low impedance, you may need a set of ZEROs to get a tube amp to work, but then it will be easy.