AR Ref 300 Vs 300 Mark II Sonic differences...

AR stands for Acoustic Research. You must mean ARC.

Besides using J-FET drivers power supply regulation is also improved.

I'm not aware of an Audio Research MKII that isn't a genuine improvement.

I would take the 6922 input any day. And remove the Sovtek tube, throw it in the trash and try out any number of tubes such as the Telefunken, Amperex PQ, Valvo, etc.

The reliability of the MK II is vastly improved.