Hey Steve, they look very much like the original tubes (I'm pretty sure they are from the photo, no brand just 12AX7A printed on them) in which case they are Chinese bottles and as old as the unit so replacing them is a very sensible idea. While there's lots of choices, I recommend New Sensor Mullards, Boogie STRs (usually JJs) or JJs in that order of (my personal) preference. (but there are lots of choices). I seem to remember there was an output level switch on the top of my MP1 ? which switched the output between guitar (inst) level and line level (or am I dreaming ?). It's not referred to in the version of the .pdf manual I have, but it also doesn't mention the rear panel input being line level (which my MP1 had) so may be for a later MP1 version ? Anyway, If I'm right and the switch is there, it should be switched to guitar (inst) level to plug into your amps clean input (pre-amp) stage or set to line level to plug into the effects loop return bypassing the additional pre-amp stage. What you describe sounds very much like a gain miss match and I suspect (if the switch is on your unit) it's set to guitar (inst) level (so no enough herbs to drive the Randall effect return, and thin, tinny fizzy noisy etc).
Cheers Richard