Is there such a thing as too much power?

You may be correct about the cause. 

The tube amps are described as: a pair of tube monoblocks using the 6B4G tube in push pull configuration. The 6B4G is a 2A3 with a different base. These amps put out around 16-18 watts. Built on Magnavox console chassis with the 50’s/60’s Magnavox transformers. The parts are industrial but quite good quality. These were built in the 90s or early 2000s and were formerly owned by Charley Kittleson of Vacuum Tube Valley. Tubes are 6SL7, 6SN7, pair 6B4G, pair 5R4 per amp. 

The other amps are Jeff Rowland 501 with separate power supplies.  The external ps make the amps sound exponentially better, to my ear, than without.

Alexberger:  I neglected to add that the speakers which recently replaced Magnepan 3.7s are Robert Bastanis Mandalla Atlas with the powered 18" woofers.  Some have said they test at 100 or 101dB efficiency.  I can't verify that though.

Stupid thread, the answer is NO.   Next!!

So what is it burns out speakers then? 
Don't quit your day job.

The 6B4G is a 2A3 with a different base.

A minor point, but for the record it's probably worth noting that the 6B4G is a 6A3 with a different base.  Both of those tubes, in turn, are electrically similar to the 2A3 except for their filament voltages and currents, which are very different (6.3 volts/1 amp in the case of the 6B4G and 6A3; 2.5 volts/2.5 amps in the case of the 2A3).

Regards,
-- Al