#1
So I finally decided on getting a pro junior (I haven't yet) only to discover by playing one in a shop in another town that it hissed and hummed like hell. The guy there told me it was just shoddy wiring of the building, but being the untrusting person I am I looked up on it online. Sure enough the pro juniors have massive hiss and hum problems. I read a way to fix it a little by backing out the middle screw on the panel, but will that be enough? The hiss was absolutely obnoxious and if fixing the screw won't help it enough I'd rather buy a silverface champ.
Winner of the 2011 Virginia Guitar Festival

Protools HD
Lynx Aurora 16/HD192
Mojave, Sennheiser, AKG, EV etc mics
Focusrite ISA828 pres
Waves Mercury
Random Rack Gear

65 Deluxe Reverb
PRS CE 22
American Standard Strat
Taylor 712
#2
I have a Pro Jr and it does hum a bit. but loud hissing doesn't sound familiar. I've never heard of the "backing out screw" fix. I'll have to look into that.
#3
"The MIA didn't have this prob but the MIM ones did as the screws they use in the back (top center) is too long and touches the chassis creating a ground loop hum, just back the screw out a bit and see, also as mentioned before, the pre amp tubes were noisy in mine, the stock fenders didn't work in mine nor did the JJ's but the for me the Electro Harmonix tubes were the ticket. I wish the JJ's had worked for me, I like their tone better but they were hummin something awful in my PJ."

from http://www.tdpri.com/forum/amp-central-station/25337-hum-pro-junior-help-required.html
Winner of the 2011 Virginia Guitar Festival

Protools HD
Lynx Aurora 16/HD192
Mojave, Sennheiser, AKG, EV etc mics
Focusrite ISA828 pres
Waves Mercury
Random Rack Gear

65 Deluxe Reverb
PRS CE 22
American Standard Strat
Taylor 712
#4
Mine had a problem for a while where it would hum louder and louder until it made a loud snap then go back to quiet. It stopped doing that lately. I don't use it a lot but I should look at getting it in good working order. I'll take a look this weekend at where it's made and if the screw does anything.
#5
Just looked at mine. It's Made in USA. It does hiss when I crank it, but at the levels I play it's not bad.