Unlikely to be a transient but only Alex knows at this point. Something with the clever way the filament and high voltage power supplies start up. You would think that rudimentary real world testing would have found this before a batch of them were made.

then again on the liquid carbon version 1, using balanced and unbalanced headphones at the same time, and the amp blows up, something else that should have been caught before a large batch were made.

 

Those pesky users doing completely normal things and the product blowing up.  They should know better!! 

Apparently another problem that has cropped up is that someone who substituted a linear power supply for the stock switching supply caused the on-off switch on the amp to go AWOL, so whenever the LPS is turned on the amp turns on regardless of the amp power switch being off. The "solution" is to ban linear power supplies. And Monoprice has no repair facilities so they refer it to a local repair shop - I wonder if they cover the cost of repairs - that's a rhetorical question, 

Well, it took a while, but this thread delivered. Surely, there's also a risk of totalling the amp if you get a brownout or a power cut, if you're driving it from a PSU with deeper reserves, too?

a number of lab power supplies have an adjustable current limit, so one of those set appropriately would be identical to the smps supply without the extra noise.

but it would be easier and a bit cheaper to build a diy version with grlv and grhv supplies, a real transformer, in a standard heatsink chassis, with the outputs transistors on the heatsinks etc.

then it would last more than 3 years. Those smps supplies when they get old, the main cap dries out, and usually bad things happen, especially to the output voltage.