If I may add my 2 cents worth, I have owned the MC2105 and just purchased the MC2125. Historically, the MC2105 & MC 2505 were one of the 1st SS amps with the blue meters and around 12 thousand of each of them were made. The MC2125 was made later and only 5 thousand or so were made. IMO the MC 2125 has a much better detailed sound and drives more bass than the MC2105.
McIntosh C27/C29 vs C26/C28 & MC2125 vs MC 2105
A question for McIntosh historians: I've owned a C27/MC 2125 since 1982, and for years abandoned them for trendier and really quite good alternatives (VTL, conrad-johnson, Musical Fidelity, etc etc) but recently went back to them and wondered about my sanity in ever having left. In fact I just added a second MC 2125, and upgraded the C27 to a C29. But in looking for used units, I noticed that the earlier generation (even-numbered preamps) are much more common, and MC 2105's must outnumber 2125's and 2155's by 10 to 1. Why is this? And is there a rational explanation for "The World's McIntosh Headquarters" valuing 2125's not only below 2105's but even their meter-less cousins the 2120's? Mind you, I'm very happy with a pair of 2125's driving Dunlavy SC-IV/A's -- I just wonder why the enthusiasm isn't shared.