Well the 2nd amp was making high freq blown horn/clipping sounds for a while, this is at 9 preamp/12 master with contour on min and tone at 12/12/12, the -10dB pad on or off. Yes it was pre-amp not the cab or power amp as this has been validated first running cab via different head, second by running tube pre-amp directly into power amp section of MB2-500.
Funny thing clicking line out/headphone switch a few times fixed this. RF feedback from power amp coming through dirty switch? First amp had sticky -10dB, which would stay on unless click a half dozen times, and noisy buffer stage (likely due to noisy transistor but perhaps something else).
If not parts to blame here
then it has to be the so-so design, mfg problems and lack of QC since products with such high partial failure rate should never hit the shelf. At this point with 2 amps failure rate is 100% even if 3rd one turns right (which is hard to expect at this point) it still doesn't look good. 
Simon: I guess some companies use their userbase to iron bugs out of their new products, perhaps they should start paying people to test their amps. And btw having tech support people to tell users that there is no problem when they call is a great way to increase customer satisfaction.
