Mesa 20/20 Deep Mod:
http://forum.grailtone.com/viewtopic.php?f=5&t=31723
Thanks for the link SC. I've got the deep mod on the 20/20, it was done by the previous owner ... I have no idea what the 20/20 sounds like as standard, but consensus of opinion seems to favour the modded version.
I'll try some different tubes for the PI positions, probably the 12AX7LPS that you've mentioned previously. If it doesn't work out for me I'll try the 12AT7, I can reuse the 12AX7LPS and install them in my SP-77 (I seem to remember Soldano recommending the Sovteks).
MJMP, under-biased power tubes are a concern ... it wouldn't surprise me if they were, I think perhaps it's a Mesa tube strategy. It's a little frustrating that the bias is not easily adjustable (trimpots, etc), so I'd be really out of my comfort zone.