MC60 has more (much more) spark on the top end than MC30 but less/rounded compare to modern MC75. The top end of MC60 is a rather controversial subject, but it is for sure more vivid, natural, and enjoyable than MC75. The HF of MC30 is round off by comparison to the other two but extension is NOT lacking, just a less forceful presentation (more of this later).
The bottom end of MC60 is deep and impactful with a nice little vintage mid-bass hump, the sub-bass perhaps even deeper and more forceful than 2015 MC75, the MC75 have minimal mid-bass hump; the MC30 on the other hand, the bass is not as deep (in extension) as MC60 also the bass imaging sits higher from the floor at 80 db, while both MC60 and MC75 's bass imaging intergraded into floor at lower listening level. If I turn the volume up more than 80db, the bass image of MC30 will connect to the floor and shake the house. It maybe due to my speakers lack of full rage at bottom end that are only rated at 34HZ. I believe MC30 can do better with Tannoy Canterbury. The Bass of MC30 "feels" lighting quick, it has less bleeding into other frequency and natural compare to MC60. BTW, I do feel the chest pumping bass with all three amps, but MC60 is a clear winner with a hammer for party monsters.
Mid range, there is no contest here, at least with current tube set in the MC60, MC30 is more forward but much more natural than other two amps. The definition, presentation, and believability are unreally good. An DIY friend visited to my house with his proud 350B amp, he was surprised he was listening to a pair Mac (MC30), he has been a Mac hater all his life before this moment. Both MC60 and MC75 high lighted the top end the same way, a little forceful than I would like (YMMV), more of hi fidelity sounding; which in the process, masked the mid-range a little bit and gave me a false feeling of recessed vocal compare to MC30.
MC60 present a mild "U" shaped presentation, where MC30 is more balanced but little roll off on HF with current tube set.