First thing: Valve/tube amps contain dangerous voltages that can kill you. If you don't understand how these kind of amp's work and want to do some work inside one you should get a qualified technician to do the work for you. In other words; just look after yerself.
here's a link to a schematic for the 1485: Silvertone
I don't know if it's original or correct.
Although it looks like it has two output transformers I would think it has one output transformer with two taps (output sockets).
Is your amp a combo or head and cab?
According to the schematic each speaker is 8 ohms which equates to 24 ohms per tap. It's kind of an odd number by modern standards but back when these were made there were all sorts of values used, I've seen amps with 32 ohms and 56 ohms required and I've got an old Rola speaker at home that is rated at 30 ohms.
So the minimum amount of 15" speakers you would need in total would be 4. You would need a 8 ohm and a 16 ohm wired in series per tap so you get your 24 ohms. I looked at some pictures of a 1485 and you might be able fit 4 15's into the cab.
Everything the Jamie said is true as far as I'm concerned. I also think that you would change the overall tone of the amp in a big way. That electrified cardboard box sound you like is probably due to the speakers breaking up, I don't think you'll retain that sound if you rebuild the cab and put new speakers in.
Personally I would go and buy a eq pedal and try to modify the tone that way for a start. It will cost you a lot less money, won't de-value your amp and save you a lot of time and stress.
