I had a Weber copper cap in my cc2x for a little while, I didn't really like it - it changed the tone at higher volumes, made it feel more "stiff" and made the treble a bit too strident where I wanted it to smooth out a bit.
I have done the treble cap mod, I'd strongly recommend that.
A search at the Plexi Palace Vox board will bring up a ton of info.
Remove bright cap on TB channel
Switch to Accutronics Reverb tank
Older models need a mod to fix a volume drop in the efx loop and to speed up the trem
Change value of plate resistors on each channel
Tube upgrade (Tung Sol RI pre's and JJ 84's for me)
Mercury Magnetics transformers
higher quality caps