Balanced narrows the field. Audio Research I/50 meets your criteria. McIntosh [whispers] MA252 and MA352 also meet your criteria.
Tube amps under $7500
Ready to experiment with combinations never before (or not recently) tried. Step one requires a tube amp. Now looking at Prima Luna EVO 400 which sells for 5K. Any other tube amps I should consider in this price/feature range? Must come in silver with balanced input. For pairing experimentally with various tube and SS preamps. Efficient 4 ohm Legacy speakers (and the room/setup) are the constants.