I bought a pair of Mk IIIs, a Red and Purple stripe, both DRGs back when you could get them for 600 apiece. After playing them, I wouldn't have paid 300 apiece for either. No harmonics, flat, sterile boring tone. Returned one and sold the other. They had lots of tight gain, but that was the only thing good about them. Ugh.
Later that year I picked up an SRG 2C. Unreal...jagged, raw in your face amp with TONS of rich harmonics. Night/day difference for me vs the 2 IIIs I had. I of course had Mike B turn it into a C+, and it was killer but damn if I shouldn't have kept it as a C. Sold it for 5x what I paid, so that was cool.
I honestly don't get anyone that somehow prefers a Mk III to a C+. Your ears must be shot. Or something. Lol
I'm kidding of course...but If you value a rich, harmonic laden sound then the C+ is on a level that isn't matched by many other amps...if at all.
The exception for a Mk III would be a Coli. They are their own animal, and are all killer no matter which version we talk about.