I love these older transistor Mac's. Very fun and musical amps to listen to. First thing I'd do is replace the electrolytic capacitors, and perhaps replace the two (the only two) signal path electrolytics in the input section with film types. Easy stuff, simple circuits to follow. This cleans up the power and helps all around with detail, punch, and imaging.
Something about these old Mac's just sounds very big and musical and low fatigue. Warm and pleasing. Perhaps just a hair too forgiving compared to some more clinical "mastering" amps, but still a great qualifier for the job. They are very underrated as well. The 2105 is well over 100 watts per channel, maybe closer to 140 or so. The little 250 or 2505 is closer to 75 watts per channel than 50. I'm a fan of these affordable classics. They go forever, incredibly low maintenance and reliable.
The ones ending in "05" are the ones with the pretty brushed faceplate and meters. The MC2100 or MC250 are the more industrial looking black/chrome style. Same amp inside.
Brad