What made you change to a 6SN7 preamp?

The MP-1 uses 6 6SN7s and 8 12AT7s.

After having microphonic issues with some 6SN7s I got lucky and got all excellent, quiet NOS Sylvania 6SN7s and great Mil spec 12AT7s for the phono stage. The improvement in SQ was dramatic and huge.

Brent Jesse and other tube suppliers do not have what private horders have.

If you can, find a horder.

I upgraded from a Cary SLP 90 (12AU7) to a Cray SLP 05 (6SN7) because 1) it was high time for a preamp upgrade, and 2) because Cary had one on their preowned page & I thought that the price was right.

I still don't think I have enough ear time into it to understand the pros and the cons

I am also still running with the Electro Harmonix tubes that came with it, and I have no doubt that those need to be upgraded, but there is so much out there I am overwhelmed with choices (so I have still done nothing)..

Brent Jesse and other tube suppliers do not have what private horders have.

What do you mean by this, I've had better luck buying tubes from vintage tube service than private sellers. Andy at vintage tube service won't sell you microphonic tubes.

Right, Andy lives for tubes. He tests all his tubes after he acquires them and adds them to his inventory. At the time of a purchase he measures  many different parameters of each tube as if they were to be used in his system. 

@immatthewj

Go to ebay and buy sylvania 6SN7 green letters and the yellowish letter tubes.  Probably between $30 & $50 for a pair each.  Start there and see how your preamp sounds.