I've owned both the IV and the LE (50th anniversary Mc275, virtually identical to the VI except for cosmetics). The LE and IV have the same basic character, but the LE has deeper and tighter bass as well as more extended, cleaner high frequencies. I ran both amps with the same tube set and got the results described above. Hope this helps.
A/B comparison McIntosh MC275 MK IV vs. MC275 MK VI
Has anyone done an A/B comparison between the MK IV and the MK VI? I had the MK IV with stock preamp tubes and Gold Lions on the output. Sound was awesome with very sweet mids, but lacking a little bass. I just upgraded to the MK VI. I feel that the bass is deeper and tauter and the highs are definitely sharper, but the mids are not as beautiful and fluid as the MK IV. I'd appreciate any thought or comments.