Re: Two generations of Class 5?
I've run my C5 head on top of a variety of cabinets (the matching C110 cab, 1965 4 x 10 cabs, 1936 2 x 12 cab) at volume and there are no issues with tubes or rattles with the head. The tubes have a retainer on them, not sure if the same as the Mk II combo. As mentioned earlier, you can do the headphone jack low power trick with the head, just flick the switch and plug a speaker cable in 3/4 of the way.
I owned a C5 combo and sold it for the head, the combo had rattles and sounded boxy to me. With the head I can use it with a variety of cabs for different tones and needs. Interestingly, the C5 head and the C110 cabinet (which is essentially the Class 5 combo enclosure without the electronics) sounds less boxy and more full to me, probably because there's a little more volume in the cab without the chassis.
Marshall 3203 Artist 30 head
Marshall 1965 A & B 4 x 10" cabs
Marshall C5H Class5 head
Fender '65 Deluxe Reverb Reissue
1977 Gibson Les Paul Deluxe
1996 Fender American Standard Stratocaster