How big of a difference do amps make?

Crucial.

I think they are every bit as important as speakers, the best speakers in the world just let you hear what your amp sounds like.
I'd rather have a $200 used pr of KEF standmounters with a 10K amp than other way around.

Contrary to Schubert, I drive my Aerial 10T with Sunfire SRA(bought NOS $450). Feels like I need a-bit more power of let's say Sunfire 300(used $700) and I'm sure that it will sound waywayway better than $200Kef with Halcro or Mark Levinson monos. At the same time I don't think that mine do well with 30wpc OTL tube amps or vintage Fisher receiver due to lack of power.

Without sufficient power to the specified load of the speaker neither price or sonic performance copmarison relevant.

Depends on the speakers. Some just can't make use of extra power. Matching amps to speakers is more important.

Cue Ralph's link to the "power paradigm"...

I had to look up the HK Citation 16--apparently it's still a fairly well respected amp (?).

Also your question is very theoretical--you'd do better asking "what amp is best for Merlin TSM's"--& there are tons of threads already on that. But yes, most people use tubes with Merlin's.

But with a "cheap cdp", your system will only sound as good as the "cheap cdp".....with expensive speakers like Merlins, it's like putting Pep Boys tires on a new BMW......