Preamp advice

Given the price limit specified and the preference for valves, you might try the Schiitt Freya + preamp. It is selectable between a passive  setting, zero gain, a "buffered" solid state stage and a tube stage (octal, 4 tubes) with 12dB gain. It is well-reviewed and reasonably-priced. Schiit has a direct sales distribution but their warranty service is good.

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Given the price limit specified and the preference for valves, you might try the Schiitt Freya + preamp. It is selectable between a passive  setting, zero gain, a "buffered" solid state stage and a tube stage (octal, 4 tubes) with 12dB gain. It is well-reviewed and reasonably-priced. Schiit has a direct sales distribution but their warranty service is good.

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!

@panzrwagn 

On the used market, Herron VTSP-3a or a Herron VTSP-360 will match up great.