ARC LS15: did they ever use Mundorf MCaps on it ?

Johnsonwu yes I am one of Steve's official repair depots. I've known Steve for a very long time. He is a friend. I also do mods but we never 'competed' for business. We all have our niches.

Most important is to get rid of the cheap bandaid .01uf bypass cap at the output if ever there's a mod attempt and you are using a premium cap there.

Do you recommend replacing the cheap .01uf bypass cap with a premium cap or just leave it out?

Just my own personal un/non-professional opinion but based on my experience with modding a few Audio Research units.
Take out the whole output cap/0.01uF bypass combo, replace with 2.2 or 2.7uF Mundorf Silver Gold Oil or VCap TFTF. No need for 0.01uF bypass. The bypass caps are a bandaid for a less than optimal main output cap.

@Ketchup check out arcdb for the schematics of your line stage. Very easy inexpensive mod. If you put a scope right at the input of the regulator to look at the raw B+ you will see why I recommended a choke.