It's coincidental.  IMO, as someone who's actually repaired several MTX
amps, they're one of the more reliable designs out there.  I suspect what
you're seeing is reflecting the fact that MTX amps are very popular (and you
also need to keep in mind the type of person who generally buys them).
There are a lot of them out there, so naturally there will be a higher
percentage of broken ones.

The answer to your question is, liquidated store returns.  These MTX amps that
you see broken on Ebay are store returns from Circuit City, BestBuy, and
Futureshop Canada. These stores offer extended warrantees and some of these
stores even offer "over the counter" exchange warrantees (no wait for repairs
warrantees). Since none of these stores sell remanufactured products, all of
these items get liquidated. There is a big company in the USA, that I can not
name. These items get sent to this company for liquidation.
A jobber (licensed liquidator) then buys these items from this company as
salvage for about 15% to 30% of the wholesale price. These items are sold by
pallets and truckloads. Due to rules that have to be followed under the
Jobbers agreement with these stores, 90% of these salvage items are sold on
online auction sites, such as Ebay. The best selling store returns are
electronics, and the above stores are the biggest liquidators of electronic
and they all sell MTX amps. You might of also noticed that there are a lot of
broken Bazooka, and Pioneer amps on Ebay, all products of the above stores,
The only reason MTX out numbers them all is because MTX amps sell more then
the others. Hope this helps.