I readily admit I've always heard a certain synchronicity when using an amp and preamp from the same manufacturer; they've always sounded just a bit better when used together than when mixing and matching.
This makes sense when you consider they were likely designed together, but it also doesn't mean you have to go that route, nor do you have to buy both at the same time.
For example, for years I had a Mark Levinson No. 380S preamp and a ML No. 334 amplifier.
I demoed an Ayre KX-R preamp and absolutely loved it (I never knew my ML preamp was a bit dark until I tried the Ayre.)
I purchased the Ayre and happily used it in conjunction with my ML for a few years.
Then Ayre came out with their "Twenty" updates, and I tried a KX-R Twenty and it was ridiculously better than the original KX-R.
My dealer also kindly loaned me a pair of MX-R Twenties while I was demoing the KX-R Twenty, and the sound was one I knew I had to have, exactly like that.
So I got the Twenty upgrade for the KX-R and also ordered a pair of MX-R Twenties.
I've since demoed a few other preamps (ARC Ref 6 SE, Boulder 1110), but none have had the (warning: audiophile cliché incoming)
magic of the KX-R Twenty/MX-R Twenty combo.
I've thought about demoing a D'Agostino Momentum Pre sometime, but at $42,500, I don't
want to know.