McIntosh MC 275 - what generation is better

Two questions to Mac connoisseurs. How one can tell latest Mark VI from Mark V? Beside green lights under tubes. Any markings on amp itself? Is there any substantive difference in the way they sound?
"Bass improved and how"

Inside the MK VI, there have been many subtle changes in circuits and component values, while McIntosh also says that its much-vaunted output transformer-winding techniques have been further improved to give greater bandwidth. This in turn has allowed the negative feedback to be taken directly from the speaker winding, giving a higher damping factor and a claimed improvement in deep BASS definition.
Hello Vad58, just curious if you ever got the mc 275 vi? I have never heard tubes in person but love what they got to offer sound wise except the heat of course. I have an opportunity to buy a pair of mc275 vi and run them in mono set up. I currently own Pass Labs 250.8  and I love the sound they create in my set up with  Martin Logans Summit X and I was told tubes will get me even better sound. 
If it were me I would make sure I audition the MC275s in my system before swapping out the Pass amplifier. You don't need as much power for the bass since its powered and the higher frequency is where the  MLs will need serious power.
Hello ottablu, yes I spent a year with MkVI but decided to part with it. I used it with Mac C22 preamp and Focal Electra 1028BEs. It does have tremendous bass which is one of the most important issues for those who want to use tubes and listening mostly to blues or hard rock. Still it lacked dynamic, or speed, or punch - whatever one calls it - of SS amps. It was so obvious when I hooked up the SS amp to my system that I decided to stay with the letter. So for now this combination of SS power amp and Mac C22 makes me happy. C22 tubes gives me the softness and liquidity of the tube sound while SS amp gives me the punch. But I think that if your listening preferences lies mostly in chamber music or vocal probably you will be totally satisfied with Mac 275 MkVI But you know - you'll never find out how it sounds unless you try all this combinations yourself. It's time consuming, it's expensive sometimes, but it worth it!