I have used used them(DN2540) and I like it so much I bought 100. It's an improvement over the traditional tube sound, more definition, improved linearity, better PSRR and these things mean lower dosage of feedback is needed. I use them in circuits where audio resolution is the priority not tube fluff. When I go hybrid I'm actually searching for a different sound one that is neither tube nor solidstate( best of both worlds)
Also I use the MOSFET to hold the tube right there in the sweet spot on the transfer curve, so if I'm correct that's more of the good stuff.
Analag