@therandyman While it does not help much in your case already owning the MX123 pre-pro, a group helping another forum member years back with in a hybrid audio / video system were using the MC2500 preamp (with HT pass-thru) paired with the MC275 tube amp for the 2ch setup, also using a separate AVR to power the rest of the speakers for the theater setup. They were going for a better synergy for the 2ch part of they system if I recall correctly.
Someone hinted at something earlier... 1) how old is the 275 of yours, 2) how many ours on it? 3) how long are you letting it warm up before listening? ...Is it a brand-spanken-new 275 amp?