The early 1970s GEC KT66 tubes I mentioned before have been fabulous, just as Golum has reported (at the price, I should have purchased a couple more for safekeeping). I've been holding off mentioning more on alternate recti tubes until I got a chance to give them some time. Not quite complete yet, but here goes:
- Marconi Osram military U52 1955 (used but tested well): Nice warm alive sound but had ringing and thus trouble with complex passages. Maybe bad example. Returned for money back
- Brimar 5Z4G -- euphonic and very good all around, but had this sense that it was sedating me rather than enchanting
- Osram U50 - this is one fabulous tube, with clarity, delicacy, articulation, speed, air, large holographic staging in all directions. Just one big negative for me -- it's not at all romantic (emotional draw); not so much cool as detached. Changing a 5V LPS power cable feeding an iFi spdif iPurifier from Paul Hynes' silver to Zenwave Audio Furutech FP-S55N copper solved the warmth problem by half.
- 596 (from dminches) - just received adapter from Bulgarian seller yesterday -- initial impression is very good all around; not quite as clear as the U50 but warmer and richer. Needs some time. Maybe the Woo Audio adapter at $100 would help it, although there seem to be different opinions about that. I'd want to be more certain that the tube is a keeper before taking that step. However, I ran into a problem with the adapter worth mentioning. The cord used is too thick and inflexible and 90 degrees off to the TRP's cord cutouts, so I can barely get seating of the adapter, having to squeeze the cord to get the adapter far enough in to seat at all. Plus it's hard against the shelf above, which makes moving the dac tricky, as well as one cord leg being up against the output tube. See the photos below. Something to pay attention to if you're adapter hunting. I have another adapter with seemingly better cabling coming from Hong Kong.