My Kustom 150 combo (pretty sure it's 4X10) gets a real hum going whenever I use the reverb. It's especially apparent when I *really* crank the reverb. It (quickly) gets to the point where the hum is more than you can take. Now, theoretically, I could just turn the reverb off and use an external box, but I'd prefer to use the built-in reverb.
I do know that the reverb does something 'cuz I can jar the amp and get the thunder sound. Fairly sure some signal is going both in and out, too, 'cuz I hear some reverb when I play through it.
I also noticed that if I shift the "head" of the combo (when I've got things all unscrewed) so that it's got the back part tilted up, the hum lessens. The hum also lessens with the back of the amp off. To me, this means that something's feeding back inside, bouncing all over.
One thought I had is that the rubber mat on top of the reverb tank may be old. I've tried to reseat it, though, and it still bends OK.