For those who have these amps, your opinion is greatly appreciated.
Here is my take on the amps you mentioned.
I own a vintage Mc275 as well as a Mc2102,i also spent about a month with the modern Mc275 in my wife's system before deciding on a Mc2102 & selling the modern day Mc275,the speakers in this system are Klipsch Heresy 2's.
In this system the differences between the 2 models of Mc275 were easily noticable,the modern version was a way better amp all around,better/stronger bass control,cleaner highs & less break up or clipping when driving the amp very hard,at very low volumes the differences are not that noticable but when pushed it was easy for me to tell,keep in mind that my vintage gear is very well kept & maintained too.
I compared the modern Mc275 against the Mc2102 in the same system for about a week,there were differences but not as large as between the modern & vintage Mc275's,the biggest difference between the Mc2102 & Mc275 was again in bass control,the extra 25 watts made a big difference even at lower volumes,the Mc2102 also clips more gracefully over the Mc275 but i attested that trait to the extra wattage.
In my opinion the Mc2102 is a much better value over the Mc275 to buy on the used market,there is not that much price difference between the 2 models when buying used & the extra performance & beauty of the Mc2102 make up for the price difference.
My brief experience's with the Mc2000.
Before deciding on a pair of Mc1201's i was seriously thinking about the Mc2000 , luckily my friend has one & constantly raved about its performance,i spent some time listening to it in his system & really liked the amp but being that it wasnt my system it was hard to decide.
We ended up taking my Mc2102 over to his house & spending a day comparing the 2 amps,we switched back & forth between the 2 amps with every genre of music we had on hand at every volume level,all througout this comparison neither of us noticed any large differences that stood out & grabbed us by the short hairs,i walked away knowing that for me any differences were very small & not worth the added cost involved.
Of the 3 amps you mentioned the best value in my mind,was the Mc2102 by long shot.
Also,as allready pointed out,as an investment the Mc2000 is not a great move,its come down in price drasticly & continues to fall ,plus its performance over other like model/wattage Mac tube amps is not enough to justify the massive difference in price,as a collectable peice it will be several decades before any return on a Mc2000 can be realized & thats if you bought very very cheaply to begin with.