Ed,
I have tried ARC VT100ii's, Bruce Moore Stereo 70, Berning ZH270's, Cary SLM-100's, Quicksilver Triodes, EAR 509ii's, Odyssee Stratos Dual Mono, and Cary SLM-70 monoblocks modified to function in triode mode and with Jensen oil filled capacitors. My favorite is the latter, as it has the greatest delicacy, resolution, and yet liquidity with vocals. There are a variety of tubes represented here (6550's, KT88's, 6SN7's??, 6C33C's, PL509's, Solid State, and again EL34's - in that order). I happen to prefer the EL34's with Quads, and the modified Cary's do it well. I am now experimenting with crossing over at 100 hz and feeding the low pass to subs. Anyhow, another recommendation for an EL34 amplifier that should work great with the 988's is the new Wolcott dual mono version of its amp. I heard it with Maggie 3.6's at CES and it sounded very good. It is great with widely varying impedence speakers like the Quad.
By the way, the Stratos sounds pretty darn good, and I've heard that the Plinius PL-102 also sounds good. I believe that.