Hey Paul,
Interesting you ask this, as I've had people ask this before. We all know that magnetic fields effect the flow of electrons (how a tv tube works).
However when it comes to amps like these the proximity to the magnet and the power tubes usually isn't a problem unless the tubes are so close they vibrate against the back of the speaker and crack/break the tubes themselves. If you have 1/2 inch of space you will be fine. Take alook at all the amps made this way over the years (lots had the power tubes very close and dead center of the speaker magnet) and they function fine. If you happen to be running some of the old metal cased tubes, it might be more of an issue as they will be more prone to the field itself.
Are you thinking about a ceramic or alnico speaker ? Alnico would probably give you more clearance. Alot of manufacturers (more so for home audio) used to put a metal cover cans over the speaker magnet to help shield the magnetic field (look at alot of old jensens and RSC speakers).
I wouldn't worry about it at all. I'd change the speaker and enjoy your amp.
Drop Ted Weber an email and ask him what his thoughts are on this. He knows his stuff on speakers and amps. I've never run across any problems with speakers and power tubes and I've been fixing, building and toying with tube stuff for alot of years.
