Hmm, expensive tubes, and just when you thought you could get off the constant, pricey, upgrade path...
Back to the matter at hand, the Def4s. These really are chameleon like, and require a LOT more work than the 2s to get right, but BOY is it all worth it when they're singing. My main downside is that I'm struggling to get my analogue to sound of it's best. My new tt with the Def2s roundly trounced my digital, but with the 4s I can't quite dial analogue in right yet, and my EMM Labs cd is in pole position, sounding just great. I'm finding that with the quantum leap in transparency possibly my cart or phono stage loading is awry, but am sure I will get there. I'm very tempted to audition the Soundsmith Straingauge cart, possibly the fastest most transparent cart on the planet, which I think may well complement the 4s nicely.
These babies are a lot more sensitive to set up - I'm finding they really like a lot of toe in, and the bass adjustments sound great with factory presets. IMHO don't be tempted to boost the bass too much, otherwise a real disconnect from the FRDs will result.
I'm happy to say I'm "getting" the treble now. Initially I felt it was still too reticent, like the 2s, but as I've got used to them, the HFs are really opening up. I love how the treble really is a natural extension from the FRDs and not spotlit at all. This is all adding to the sense of realism across the board.
My only slight feeling of disapointment is that they sound a little more subdued than the 2s at lower volumes, puzzling since the efficiency rating is still 100db+. I am finding the need to turn the volume up a little more, which is an issue during late night sessions. It may be that the gains in transparency and detail is slightly offset by a decrease in density.
Will post further as get to know them better.