Ah how those old McIntoshes sound! I'm happy to have one myself, albeit a 240.
Eventually I am going to split my stereo up into 2. The Electras will stay with the Koras but for the 240, I am going to get a pair of Klipsch Cornwall III. Awesome sound, if not the most svelte looking speaker. Any of the Klipsch Heritage speakers would be great matches.
I personally feel that if you what to hear what this amp can really do, you need efficient speakers. I have "been there, done that" with mine and efficient designs allow its resolution to shine through way more than tough loads do. It can sound great with "regular" speakers but it won't be as good as when it has headroom to work with. Though I also have to say I like to crank it fairly often...
Options that would allow it to sing IMO, in no particular order:
Coincident (all models)
Reference 3A (all models)
ProAcs (smaller models)
Omega single-drivers (all models)
Cain and Cain (all models)
Zu (all models)
Triangle Comete ES and Altea ES
Reimer (all models)
Cabasse Java 350 and Farella
Living Voice (all models)
Tannoy Glenair and Reds
And if you really want to be astonished by your amp, send it off to have a complete overhaul. Those 40 year old caps are worn out and the bias resistors are probably out of spec. Not to mention getting rid of those selenium stacks for some latest-technology diodes (FREDs in particular). Trust me - you don't want a big cap to fail during operation! Besides, it will restore its power output capability, and probably even gain some. And since you've just done all that, you might as well tweak it with some small MKP filter and bypass caps on all the electrolytics. I rebuilt mine and it is now the most musically transparent amp I have ever heard.
Oh and one last thing: have you tried turning up the gain rather than using them at the dot indicator? I find the sound of mine to be clearly more dynamic when the gain pots are turned up to 2 or 3:00 rather than 12:00. Worth a try if you haven't already. Beyond that point, a little hardness develops on mine.