I think going with the ToneSluts was a good idea.   I did the same thing, but went with Heyboers.  As well changed the stock Orange Drops to the Sozo Vintage Series caps. Sprague Atoms for the power caps.  My Expression stock was amazing in the Cleans, but the dirt was harsh to my ears. Tried all types of nos tubes. Nothing worked. Changing to Heyboers and the caps listed above,  all hint of harshness has disappeared for me. And the amp got quieter.

 Tried the Tungsol 12aX7s, but went back to all JJs. My changes just made this Expression more like what you hear from an actual Express. Sweeter, more musical. Because Full Power is a tad loud, I use it at 1/2 power atm.  Plenty of kick for jams. Using a Bogner Cube loaded with a Scumback fullbreakin M75.

I find I am not using the PPIMV at all. Turning it down doesn`t push the power tubes as hard I find.  It seems the Wreck Magic is because of the combined preamp and PTs working together.  My Volume at almost 2 o:clock. and Master maxed seems to be the sweetspot. Atm, I set all the other EQ knobs between 11 and 12 o:clock. The rest done from the guitar.

I have a EMCT Bartlett 59 Replica wired 50s style.  Compared it to my `78 and `71 Les Paul Customs wired stock, the 50s style wiring DOES seem like the better match when paired with my Ceriatone Expression.  Oh, I can coax sweet tones out of the Customs too, but something truly magical happens with the 50s wiring.

                                        Should add that I am using a Zakk Wylde Signature Black Label EV in  a custom made Thiele cab.  BIG full sound.  Surprised the crap outta me when I paired it up with the Expression.

Just got my Expression, and can`t seem to find this switch. Is it located inside the head cab? Thx.

Gary

addendum:

 found the switch, it is on the inside of the cab.