Kats -
To me, it seemed like the HiFi tuning fuse made a difference. I wasn't expecting to hear a difference but thought I did. I felt this was confirmed when a tube went and took the fuse. I had to use an inexpensive Radio Shack fuse until I got a new replacement HF fuse. Without the HF fuse, it seemed like the music lost a little "liveliness" (maybe some air and detail?). This was restored when the HiFi fuse was replaced. Placebo effect? Maybe. But for $30 bucks, and even if it is placebo, if the music sounds better or even if it just seems to, it's an inexpensive "tweak". Search the forums on HiFi Tuning Fuse (there are other upgrade fuse options, as well). There are definitely naysayers but I'm not the only advocate.
BTW - I switched from 6550 to various KT88s. Lastly running a quad of Winged C KT88s. I thought these were really great. Also, see if JoLida is still doing a factory mod/upgrade. That was ($200?) money well spent.
Finally - I've no commercial or business interests/connections with JoLida. They just made a great affordable product (my intro to tubes) that gave me a lot of enjoyment for a decade. I recently retired it in favor of some mono-blocks but have no intention of letting it go. I'll use it as back up if I need it.
