If the modules don't make good contact, the amp doesn't work. You just have to use a little contact cleaner on a Q-tip and clean the contacts. I also used a jewlers screwdriver to bend some of the little copper tabs in a little so that were a little more snug.
Not a big deal really - took maybe 1/2 hour to do both amps, and I've only had to do it once in the 3 or 4 years that I've owned them.
They're also very particular about rectifier tubes. I run old Sylvania 5u4gb's - any of the new manufacture Russian tubes I've tried have blown after a few hour's use, but the Sylvania's have been going strong for quite a while now.
Both of these are known problems, and were pretty easy to research on the web. I think that between them, the account for about 90% of the "unreliable" reputation that these amps have gotten.
I guess, at the time, they were a pretty expensive amp, and nobody wanted to drop that kind of $ on something that might have problems. They sell for about $500 now on eBay, so that's pretty affordable, and good work-arounds have been figured out for their major bugs.