I purchased my 501's in 2006. After listening to the MC 252, 352 and 452 amps, I decided I wanted to go mono with a little more power. When I was ramping up to purchasing these, myself and others were having conversations online regarding the design and power aspects, as well as real world capability.
Although I can't find the exact post (from 14 years ago) it was discussed between a number of us and Ron Cornelius at McIntosh, that the 501 fed a clean 120 volts on a 20 amp line is actually capable of over 700wpc. Redlighting Power Guard at 50% he claimed closer to 800 wpc during laboratory tests.
I have no way to test those numbers, my dummy loads are only rated to 500wpc, however there was no reason for Ron to lie to us, let alone on an open forum. My experiences with the 501's are that they can run any capable fairly efficient speaker to insane levels in a 16 by 20 foot room with 10 foot ceilings. Levels that are not painful because distortion levels are low, but will leave your hearing with a ring for several days after,..levels as loud as any rock concert I've ever attended.
I know in the real world people own a variety of speakers with different specs and can be difficult to drive. That said, I don't find the sound/performance abilities of the 501's dated or limited. Personally, I don't see any need to upgrade ever, but then I'm that guy who still owns and uses multiple 2105's, 2205's 2255's and 2125's. With some new caps and other minimal age related work, those amps sound as good as new today, perhaps better than most of what is on the market today, to me anyway. Unless I hit the Lotto or whatever ( I don't ever play, so fat chance) bigger more powerful Mc amps aren't in my future.
I'd much rather keep my preamps and digital audio (CD, DVD, SACD, DVD/A, Blu Ray, High Resolution, Streaming) units up to date with the latest performance levels you can hear, buy a lot more vinyl, and keep upgrading my turntable, (I just bought VPI's Prime Signature this year), as opposed to chasing more watts. I'm similarly contented with my speakers,.. Mc's own XRT 22's which I've owned even longer, over 20 years, which continue to sound perfect with new capacitors I recently installed into their aging crossover networks.
How many can honestly say they are that content with the sound and quality of their gear after 14 years or more? I happy to say I am one of those people, and proud owner of several 501's.
What these amps do need are those glorious and gorgeous rack handle upgrades that Master Liu offers for sale, which makes the "little" 501's look every bit as formidable as the bigger dogs in Mc's amp stable..
If you are ever in the Hanover Pa. area, feel free to stop in for a listen...