One month ago I purchased a McIntosh MC-275 Mk IV to replace my MC-250 for two-channel and home theater listening. Terry DeWick restored my MC-250 to specs last year.
Back in 1970 the MC-275, 75 watts/channel, sold for $444 and the MC-250, 50 watts/channel, was $379 (the MC-2505 sold for $449, the blue meter version of the MC-250). Based on the similar prices one would assume they would sound about the same.
Listening test:
I used Chesky’s
Ultimate Demonstration Disc to test the amps. The Chesky CD is well recorded and easily available, and I have spent many hours listening to it. I have listened to many other CD's in the past month. The system consisted of Klipschorn’s, Cambridge Audio Azur 640C, Denon AVR-3200, and Monster cable.
High resolution: Spanish Harlem. Rebecca Pigeon’s voice was very distinct and filled the room, the MC-275 seem to put a ‘halo’ around her voice. Heard more of a sharper distinction in the vocals with the MC-250.
Edge MC-250. Depth: If I Could Sing the Blues. Virtually identical. Placement of the trumpet is identical, about 15 feet behind and to the right of the microphone.
Tie. Midrange purity: Grandma’s Hands. Near the end of the song you can hear tapping of feet, seemed more distinct with the MC-275. With the MC-250 sounded like they were wearing tennis shoes, with the MC-275 they might have been wearing dress shoes.
Edge MC-275. Holographic Imaging: Festival Te Deum. You certainly can tell this piece was recorded in a large cathedral. No edge to either amp.
Tie. Bass Resonance. Maybe more detail in the MC-275, especially as the fingers moved down the strings.
Slight edge MC-275.
Appearance: Edge MC-275. Who can resist the glow of tubes?
Measurements:
Measured the room response curve of both amplifiers using full range test tone from Room EQ Wizard. The MC-275 seems a little flatter in the higher ranges and does a better job at 12-17 KHz. Lower end the curves track real close.
Edge MC-275. 
Overall: Slight edge to the MC-275. Conclusions: The newer MC-275 Mk IV beat the MC-250 by a small margin in my very subjective tests. Is the difference in price between the MC-275 at $3,900 and MC-250 at $450 (after a going over by Terry DeWick) worth it? The MC-250 is one of a value. Very warm, tube like sound at 1/8th the price!