Depends On Which Inputs You Are Using
If you are using the unbalanced (RCA) inputs V4 is not in the circuit at all. And I find I get better imaging using the unbalanced inputs.
BTW you will see references on the audio boards claiming that the opposite is true, but this is opinion based on earlier versions of the reissued MC-275. I have also seen were McIntosh claimed that the Mk VI is using the same circuitry as the earlier versions, which seems me to be inaccurate.
I was hoping to attach a screenshot of the block diagram or schematic for the LE/MK VI but sadly I dont see how to do it. But both clearly show the left balanced signal path going thru one section of V4 and the right balanced signal path going thru the other section, before heading on to V3 and V5. And in the Service Manual McIntosh refers to V4 and the circuitry around it as a "Balanced Amplifier". The unbalanced signal path avoids this part of the circuit completely. If you would like to see it, PM me and I can send it to you.
So my advice would be to use the unbalanced inputs, use your tubes in V3 and V5. Put any working 12AX7 or equivalent in V4--as it may be needed to complete a heater circuit--but it will have 0 impact on your sound quality.
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