Buzz/Distortion on my Princeton Reverb

I want to ask for some help with my '82 PRII. I've had the amp for about 3 months now, practiced with it a number of times and gigged with it twice. The last time we played I thought the amp didn't have enough headroom to the point that it sounded shrilly when I crank it up to stage volume level. I attributed it to speaker distortion. I know I'm probably off here but anyway. So I decided to change the original speaker with an Eminence Legend GB12. After I have changed the speaker, I started hearing a buzz/distortion when I play the open E string. It also only starts happening when the tubes get hot (I usually leave it on for at least 15 minutes). So I thought maybe it's just because of the speaker. So I put the old speaker back on and thats when I realized it does buzz too, but not on the open E but when playing the B note on the A string. Changing the setting on any of the knobs except for the volume control has no impact on the buzzing sound which actually continues on indefinitely even after the string has stopped ringin. Talk about sustain.

Might it have something to do with the enclosure resonance that affects the speaker? We're going to be trying some other speakers but like I said I can hear it with the old speaker as well, altho when I tried it, it was out of the cabinet.

Any comment/suggestion/help is appreciated. Thanks!!