Well. Each amp is its own thing. The Quad Pre has BOTH preamp sections - Mk. III and IIC+. So they must be different enough for Mesa to include both as distinct sections rather than choosing one or the other.
BUT!
Their definitely in the same constellation of tone. To me - they sound very similar. But if you get a Mk III hoping to nail a IIC+ - you'll be disappointed - get a IIC+. If you get a Mk III for what it is (which is badass!) - you'll be stoked!!!
The markIII has similar tonal "stuff" that happens when you play through one .
But the " StuFF " that happens in the IIC+ Is different and happens in a different tonal universe . UGC RP11 in the galactic catalog .
Luckily only some of it leaks through into our time line or people's heads would explode .