What type of tubes are in there ? NOS tubes are the way to go ! The new-old-stock from Philips/GeneralElectric/RCA/Mullard/Brimar and many others, will often sound dozen times better than any currently made ones (chinease/soviet). You will just be amazed of the difference in quality between them. I use a combination of GeneralElecric and RCA 6L6GCs, a (probably) british NOS 12AX7 labeled "Lewis & Kaupfman", GE and RCA 12AT7s in my 135W TwinReverb.
seriously, though, any tube amp is going to be built in such a way as to make it not THAT difficult to take tubes out, especially from a company like fender, who knows what tubes are. is there some removable trimpiece around it? the metal box's screws could be under it. other than that, i don't know. i don't think i'll bump into one anytime soon, or i'd check.