Hey Stuart,
I did try out a few tubes with the MP-D2 namely the Russian 6H6p, Radiotechnique 12AU7, Reflektor 75' grey shields with single as well as plate getters, Siemens E82CC and the 6H30Pi gold pins. I will quick list a few traits of each tube ( IMHO). Mind you...all of the above are pretty good in their own way and in a properly matched system would shine....
I usually listen through my studio monitors which are PTE Phoenix ( as good or at least approaching the ATC 50 ASL's - again IMHO). I use a Audio-Gd Master 9 as my preamp/headamp. I use balanced cables which I built myself with 16agw occ silver for the signal wires and occ copper for the return). I use Shunyata Hydra's for power conditioning and Synergistic Black fuses all over. My system ....for lack of a better word ...can be called as ultra-neutral. And this - to a large extent- has also influenced my choice in tubes - I think.
The Audio-Gd adds a slight bit of solid state character to the sound of the MP-D2 but otherwise is quite neutral. So here goes...
1) Radiotechnique 12AU7 - I think these are the most neutral and extended 12AU7's that I have used. They are well extended on both ends of the spectrum, have a tight bass, decently wide sound stage and a very good mid-range. By themselves, these are probably the last 12AU7's that anybody would ever need. Very quiet.
2) Siemens E82CC - I have read that these are supposed to be slightly rolled off in treble. While I don't think that they are particularly rolled off in the treble, they do have a slight emphasis on the midbass and bass which tends to flavour their sound signature a tad bottom focused. But for someone with using bookshelf speakers or if your system is slightly lean or using HD800 headphones - these could very well be the tubes. They have just the right amount of extra warmth and slam to fill up the sound.
3. Russian 6H6Pi - Very good tubes. Very wide sound stage, well extended top end and a gorgeous mid-range. They have a good mid-bass but in my system the bass sounds slightly light compared to the rest of the group. It a difference between A & A+ but still... I would recommend tube dampers for these. Makes an improvement.
4. Reflektor 6922 75' grey shields - These tubes have it all. Extension, tone, soundstage, mid-range, vocals.... everything. They are probably equals to the 6H30's in every way. The only place where the 6H30 overtake them is dynamics and bass slam and that too, only by a very small margin.
5. 6H30Pi Gold Pin - ARC Select from Upscale Audio - I think these tubes deserve the super-tube name. They are super extended on both ends, have a wide 3D soundstage (largest of all tubes above), fantastic mid-range (vocals, vocals, vocals...), are very quiet, very detailed. But the the best feature of these tubes is their tonality. Violins, Cellos, Guitars, Pianos, Trumpets...all sound heavenly through these tubes. And then of course; there is that bass....punch in the gut kind of bass. This tube is also very very linear - almost ruler scale flat frequency. I think my money was well spent on these tubes.
Another tube I tried with the MP-D2 is the Sylvania 6201 Gold Brand (12AT7). I think this is also a very special tube. I did not get to listen for very long as they belonged to a friend but this tube has a very sweet and very very well extended and linear sound. What really stood out to be was how quiet this tube was and how pliant and extended the bass was on this tube. This is another tube which is on my 'to buy' list.
I hope the above description helps. I do have a tendency to get carried away..my bad.
