Hi Adam; PM sent re/ the schematic. If you have CTS speakers identifiable by their original plastic bells or, if gone, square, flat magnets about a 1/2" thick and bear 137 as the three leading digits in their EIA code, they are ceramic.
Your amp runs 6GW8 tubes; similiar only to, but not directly replaceable by, 6BM8s; both are an unusual combination of triode and pentode sections in the same bottle. There are new old stock (NOS) domestic 6GW8s however this tube was chosen by Guild as much for its operational characteristics as sound quality. Coastie, one of our BB members, has a considerable stash of 6GW8s - if you want to do a little 'tube rolling', he'd be the man to get in touch with. I happen to know that he has pair of killer Tungsram 6GW8s ... but assuming he'd let them go, it might cost you several cases of Heinekens (called 'boxes' where he lives).
If you like the amp and 'tube rolling' doesn't get you what you want, you might consider contacting another BBer, 'default', and talk to him about re-wiring your output sockets for 6BM8s. The 6BM8 is back in production from Electro Harmonix and there are really great-sounding used Euro versions of this tube available; Amperex, Tungsram, Mullard, and Telefunken. There are also Euro 6GW8s (although fewer of them) which, to my ear, are more pleasing than domestic ones. However to use 6BM8s in the Thunder amps, there are 4 wires on each output tube socket that have to be relocated. If I had a Thunder series amp, I probably wouldn't hesitate to make this change.
If you have a 3-wire power cord, the polarity switch doesn't matter; in fact it should be disconnected altogether. If you don't have a 3-wire power cord, for safety's sake, that's next on your list. Polarity switches are left over from the days of ungrounded (2-wire) house and commerical power wiring; if it hummed in one position, it was switched to the other to kill the hum instead of yanking the plug from the wall and turning it over.
On the speakers, since my interests run to farty old box amps with alnico speakers, I don't have any suggestions. But if it were my amp, I'd tug the speakers out and check the cones for rips, tears, holes or general softening ... they can get limp as paper towels. Good luck! CJ