I've had the Fosi BT20A-Pro for a couple of years now. Initially it was a quick and easy replacement for my 70s Sansui with blown output capacitors. Eventually it made its way into my office, so I use it for everything from conf call audio to YouTube videos to movie watching to streaming music. It's always been fine. Not spectacular, but fine.
Anyhow, I decided to plunk down for the MUSES02 and upgrade my op-amps. For $52 or so, it was a pretty safe amount to spend -- especially if the difference was negligible at best. Swapped the chips (they're really small), resoldered one of the speaker connections that came off, and went back to what I had been listening to.
OMG!
There really, truly is a noticeable difference. With my setup, I immediately noticed how much more punchy everything sounded -- even with no burn-in (yet). There is a presence that simply wasn't there before. I finally feel like I'm doing my 80s'era Allison CD speakers the justice they deserve ;)
If you're thinking of going from stock op-amps to the MUSES02, absolutely go for it! I can't speak to the other op-amps, but these were absolutely worth the $50 and 10 min upgrade!
