That's huge that the C27 needs a power amp to activate the headphone jack.
Thanks for the advice! If I can swing both, I'll get both the MC250 and C27. Otherwise, looks like I'll lean towards the 1900.
As a person who uses both, I guess I should chime in with my 2-cents.
I think part of the decision would be the condition of the units - unless you have plan and budget to have them electrically and functionally restored after purchase, I'd favor units that are already restored - hopefully restored by a McIntosh specialist or expert tech.
Since all the units you're looking at are really old, their sound quality will be determined by their electrical condition. Sounds like the MC250 is "fully restored" but if you get a C27 or a MAC1900, they should also be fully restored - else these sound comparisons are meaningless.
The MAC1900 is more complicated to restore as it includes an AM tuner, FM tuner, preamp and amp.
Many McIntosh units from that time period used a power switch combined with the volume control such that every time the unit is switched on or off, the volume control is rotated from the off position to the normal volume position which wears out that section of the volume control prematurely. The result is a scratchy sound when adjusting the volume and also left-right balance changing as the volume is adjusted.
This worn-volume control issues seems to be un-fixable. I have watched threads on this forum of people trying in vain to find a replacement part that works or even rebuild the existing controls. I don't recall anyone reporting success.
The MAC1900 is one of these units so
before you buy one, make sure the volume control works at least well enough to be useful as is. Restoration won't help this. C27 has a separate power button so not a issue.
Cosmetics are hard to restore, so you want to buy stuff that still looks good, especially no cracked glass.
If the units are working to original factory specifications, then my opinion is that the choice should be based on condition, features and even looks. They both sound great to me.
Good luck.