Seems like there is a lot of conflicting information on mismatching loads on amps. From what I've been told, mismatching the load as you did will create a more compressed and lower output tone in a tube amp scenario (i guess that is why some players like it, it makes an amp break up earlier, as well). That compression may account for the loss of high end sparkle on the mac.
For the system to be most efficient and to put no strain on components, it seems like matching the impedance is in general the best idea out there. Most OT's can handle the load mismatch, and in most cases your gear will survive, but why test Output Transformers and wear out tubes prematurely, especially on a vintage expensive piece of equipment. I have no idea how mcintosh amps handle this. Supposedly Mesa's are safe to do this with, Fender's can handle a mismatch one up and one down (lots of conflicting ideas here), and marshall's can't handle the load mismatch at all. so who knows on a mac?
like waves against the bandstand dancers broke