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

Best of luck to the OP.
If you like the Audio Research sound, not to mention their great support (we here are spoilt by the excellent support from Leonard in the past and now Kalvin), then perhaps saving some $$ and jumping up to the reference series will save a lot of $$ lost in incremental trade ups.
LS22 / LS25 are balanced design. Do you use balanced cables from source to power amp to take advantage of that?
Yes they will work with SE source and power amps connected with RCA but yo uwouldnt be taking advantage of the design.
They are very dynamic sounding but overall not my cup of tea.

Both versions of the LS15 sound different than the other, the LS25 and LS25ll are also different. They all have different sounds, not simply better or worse. None are worth buying without a listen.

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.