I've found the Audio Research Reference 6 to communicate an unsurpassed level of presence and three-dimensionality while remaining tonally neutral. Class A all the way -- with only some reservations in the high frequencies: a slight lack of richness and sparkle, most easily heard with piano.
I've found several tweaks, when used together, enhance the high frequencies of the Reference 6: first, a Synergistic Research Black Quantum fuse; and second, a Sain Line Systems Pure Current power cord. Those two tweaks on the power-supply side get you 80% there. As a final note, I swapped in a set of matched, cryogenically treated, low-noise 6H30 tubes from Upscale Audio and an NOS Winged "C" 6550C power tube. Now, class A ++, with no reservations.
A new Blue Synergistic fuse is on the way.