Muso or UQ?

I would have thought the UQ would be in another league altogether over any Musu provided it is matched to the speakers correctly and also setup right (positioning for the room etc).

I suppose those are two things that can go wrong whereas a Muso is pretty much a plug in and switch on affair and as long as it is not in a tiled corner should sound good.

Cannot say about the Credo's. They would not be the first (second or third) speaker that springs to mind with the UQ. On the other hand, they are not terribly difficult to drive so why not. They may be more revealing a speaker than you want on a UQ. I would first review the setup and build of the Credos (have they ever been rebuilt with new gaskets?). I would also borrow a lower cost speaker and see whether you suddently get synergy back. I use a UQ2 with Qacoustics 2010is (so not even on the same planet as Credo's but maybe 3050 at GBP600 would be just the ticket) and this sounds better than sticking my PMC Twenty.23s on it which need a different level of amplification entirely to get out of bed.

I've yet to hear a mullet that sounds good (though I've had to listen to many friend's mullets and lie and say "sounds great dude").

What format music are you listening to? Ripped CDS? Mp3s? Flac? Wav?

There are a lot of variables but I'm surprised that you feel the Muso is preferable in many ways.

The Credo is not like the SBL in that it does not have gaskets - the top box is separated with little pins - so there is really nothing to go wrong. 

As others have said, the Qute setup should sound a lot better, and the Credo is easy to drive. I agree that it's worth borrowing some alternative speakers to try. It could be that the Credo crossovers are knackered, or the speakers have some other fault. Check that the soldering in the speaker leads is still ok, and that if you are not using NACA5 that it is compatible with the Qute.