Thanks for the clarification and sorry for the oversight! Mc model codes can be tricky in the amps/preamps section if one isn't paying close attention.
Regardless, the MC2500 and MC2600 are generally considered the two most "classic" McIntosh solid state power amps ever introduced. The MC2500 was the foundation of the "Wall of Sound" during the Grateful Dead stadium tours back in the day. They used a truck-full of them at every show and never blew one up. One of McIntosh's claims to fame today: They have never had a catastrophic amplifier failure reported.
Differences? 500 WPC versus 600 WPC. Identical architecture internally. Almost identical cosmetics. Bullet-proof reliability. They made more MC2500 units than MC2600, so the 2500 series is more readily available. Similar pricing on the used market.
I've not heard the MC2600, but the MC2500 set the standard by which all other SS amps were measured at the time and I found it to be detailed, effortless and quiet. I can't help but have high confidence that the MC2600 would be the same.
If it was my money, I'd probably try for the 2600, just because it's the rarer piece and I haven't heard it. If I didn't like it, turning it around on a 2500 would merely be a matter of time; losing money on either unit is nearly impossible.
Good luck & happy listening!
