I used the MC275 with Harbeth M-40.1's a few years back and it was a nice combo. I have no experience with the Leben, but I'm sure you'd be fine with either amp.
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MC275 has a more gutsy powerful presentation and it is also more fluid. The Leben has better micro dynamics and micro details. It has a certain sparkle in its sound which is makes for a lively presentation.
If your speaker needs more power, get the MC275.
If you listen to Rock, pop, blues, big band more then the more gutsy and driving sound of MC275 will be more enjoyble. If you are into small ensemble Jazz, Acoustic music get the Leben.
I have owned both at different times. Both are worth demo'ing in your room with your equipment and will give different results. Remember your hearing is as different from others as your fingerprints, as is your room & equipment. The Leben is exceptionally well built & has a really great headphone output for late night listening and the ability to run different output tubes if that makes a difference to you.
I heard both amps, with different speakers though. I listen mostly to Jazz, vocal and Pop. It was a hard decision to make. At the end, I went with the CS600. The headphone output is a plus. My Sennheiser HD650 sounded very good with it. The CS600 came with stock tubes Sovtek 6L6WGC which I didn't like. I replaced them with new reissue Mullard EL34 and I like it much better. The Mullard EL34 sounded less lively (with less extension on both ends) than 6L6WGC but it is smoother and the mid range is more pronounce and sweeter. I can live with it. I will try other tubes such as KT77, KT66, KT88, KT90, KT120 and 6CA7-Z (maybe this tubes is compatible, got to double check) in the future. I just noticed that the price for SED Winged C EL34 at thetubestore has increased again. It was $49.90 last month. Now it is $57.95. Probably I should get a quad now. Some say KT77 is better. Some say SED Winged C is better.
Just to let you guys know that I have tried Sovtek 6L6WGC, new reissue Mullard EL34, 6P3S-E, JJ E34L and EH KT90 tubes in the CS600. In my system, the KT90 sounded the best and most balance among all these tubes. The warmness of KT90 complements the natural and neutral sounding M30.1 speakers. The KT90's bass is extended, solid and fast but not too strong. Female vocals sounded sweet. The KT90's high is just right, not too extended, smooth and never fatiguing. No wonder Jeff Day likes the KT90 (http://jeffsplace.me/wordpress/?p=1866). I am very happy with how my system in sounding right now. Not thinking of changing anything for now, at least for a while. One small downside I wish KT90 has more vocal's baritone. If you know any tubes which sounds similar to KT90 but has more vocal's baritone, please chime in. Probably, Ill try the KT120 in the future and tube rolls the smaller tubes.
Going on measurements, if the CS600 is anything like the CS300, I would opt for the MC275, it has the far better looking measurements re FR flatness and square wave ringing responses.
The Leben rings like a bell with the square waves and is also shown by it's response which climbs like Everest after 10khz.