I use to install MC 75s for 3 channel Klipsch systems, to drive fill speakers, and 70.7 volt lines where Altec 1569 amps just weren't good enough. The 75 was rated a .3 distortion where the 275 was rated a .5% distortion. The difference was the 75 had a much stronger power supply in comparison to a 275. Now a MC 30 with those tube rectifiers has a special sound. You either like it, love it or its nothing special. A 75 was a great amp. Almost 100 watts per channel before clipping with great signal to noise and low distortion. It didn't have that clarity the Marantz model 9 had, but i thought the 9 was rather austere. The 75 wasn't as smooth as a 30 was, but then nothing was. I much preferred MC 225's. I thought it was Macs best sounding amp till the 3500 came along. I'm talking about quality not quantity. But if you are after that special euphonic sound from a special tube unit then you should be happy with your 30's
Now I say all this trying to remember what Mac amps sounded like with Mac provided tubes. Don't ask me about what the sound would be using Mullard or Russian/Chinese tubes. I have no clue. The 75 is going to sound a little cleaner and dryer than 30. I think thats because it just does a better job of being faithful to the source. That can be a good thing or bad, maybe your speakers need to be tamed a bit. KEF and Tannoy really need taming. Where speakers like older Altec Carmels, JBL Harkness could use a little help from a 75. MC 30's and Bozak aren't my favorite match, give me that smooth little 502 SS amp. Folks who found EV DH1 and DH2a drivers a little edgy would love a MC 30. Where some speakers like Macs own ML1 would much prefer the 75. Altec 802 drivers and 288 C though not needing the power I feel would be better served by the 75. Bozak Symphonies with the 8" mid loved 30's but they came into their own with the 75.
Its all a matter of taste I'm afraid. Being a SS guy thru and thru, I would rate the 225 first the 75 second and the 30 3rd on neutral speakers with great tweeters. But if your after a special sound that reveals audio characteristics of live recordings in large halls or outdoors a MC 30 has a special gift in producing that space. Where a 225 might have brought you a little closer to the musicians and the 75 better reproducing the dynamics of the same recording. The 75 certainly will have more authority and the 225 better able to define the signal , be that good or bad. Remember tube amps with low damping factor can be more easily influenced by the reactive components of a speaker system versus amps with very high damping factor. So while a MC 30 might be the best choice for speaker A and poorest for speaker C a 75 just might be the inverse or a good compromise. its all up to you.