XLR??? IIRC, Ampeg used that on some of their old V-series cabs for the safety interlock. Is this a Peavey 5150 cab?

Replacing the power tubes usually means that you need to re-bias the output stage to match the new tubes. You could try stuffing the old power tubes back in, just to see.

I think you need some advice from someone who has one of these amps. Shoot a PM to PRSplaya and see what he thinks.