I can't help with speaker's much as I've never replaced one in a guitar amp.

The 12" Sheffield in my Peavey is pretty good. I'm sure I could get a better one if I started swapping different types in there, but I usually run that amp into my 4x12 anyway.

The speakers in the quad are 16 Ohm 12" jobs that are about 40 years old. They look like this. That's not my rig, but I have the same box in black.

Anyway....soldering irons huh