I also run a MC275VI with modded Cornwall IV's. I tried a number of tube pre amps starting with a C2300, then C500T, both were blown out of the water by a Modwright LS100. To say I was shocked is an understatement. I could have easily lived with the LS100, but in my large dedicated listening room, I found the lack of a volume readout to be a PITA. I purchased a Don Sachs II from Don just prior to him signing on with Spatial Audio and my pre amp journey is over. It is an outstanding match. Uncharacteristically there are 2 or 3 "used" DS II's on the market currently as some users have moved up to his new offerings thru Spatial.
I also auditioned Schiit Freda+, Rogue RP9, Aesthetics Calypso. Didn't work for me in my room.
Enjoy!