What sound diff. can I expect from PrimaLuna EVO 400 pre and power amps VS an EVO400 int.

Going separates will get the last bit of refinement the integrated can’t produce.

Integrated if you just want to enjoy good sound and not be neurotic about audio.

Speaker choice and placement will make or break your expectations.

The phonostage will handle a high output 2.5volt MC. Get a separate phonostage, if you’re serious about LP playback.

Longtime PL user.

Since you're open to used, I'd look into something on this level

Manley Chinook tube phono preamp For Sale - US Audio Mart

tubed, good reviews. I demoed one in my setup.  These can be had for less, if you're a negotiator or patient/ready to pounce on better price.

The Mani is great for what it is-a $150 phono amp. I had one as a backup while getting my proper unit repaired.

"I am using a Willsenton R8 integrated Chinese-made tube amp with KT88 tubes "

The PL will have noticeably better construction details/parts, but It may not be leagues "better" in SQ?

Based on things I've read, it's a decent amp-for the price. Just how much "better" the PL integrated would be something between you and your ears?

That's where separates AND your wallet will make a significant difference.

The bad news you should already know is it always adds up to the sum of ALL the parts.

The volume control comment is for the audio neurotic category.

That volume pot is used in other popular "good" brands for some time. Ridiculous statement to make to not consider choosing it.

There will ALWAY be "better" ways to implement things.

That said, the PL reviews both "pro"/ end user have remained positive for many years.

EVERYTHING can be picked apart, including esoteric ridiculously priced gear.