I've been there once, had a problem with my subwoofer Bowers and Wilkins ASW610XP , an issue with the power supply board.
After a looong wait, they told me they couldn't do anything to repair it, they told me the the Board is not repairable at component level (we are talking power supply here, nothing v complicated), and couldn't get the whole PCB from b&w either.
in the end I lost £40 for the "diagnostic check"and also the ubers I had to pay to carry this monster to their shop.
I guess if you need a service check and maybe just the dust removed inside that's probably fine.. They seemed like nice people, but for anything more technical I can't say I had a good experience.