McIntosh C-22 Re-tube

I am very fortunate to have a hand-me-down vintage C-22 with its classic partner MC-240. I have extensively tested these units with each other, and each with other partnering preamps and amps, and have come to the conclusion that my C-22 just does not sound good. Both have been 'refurbished' (lightly) by Audio Classics. So they work. The MC240 has been re-tubed with Russian tubes, and generally sounds pretty good in stand-alone tests. The tubes in the C-22 are Chinese, I think, standard with the Audio Classics "tune-up." Question is, is this a simple matter of buying better tubes for what I understand to be a great preamp, or is there some other variable I should have looked at within the preamp that could be causing it to sound lousy?
Partnering equipment in my sound tests were B&W 804 Diamond and Spendor FL-9 loudspeakers, Audio Research 100.2 amp, and Bel Canto Pre2 preamp. All components (to include loudspeakers) were tried with every permutation of equipment pairing. The C-22 continuously turned up as lacking.