Is there such a thing as too much power?

Atmasphere, Is there a way to determine an amplifier's lowest point of distortion apriori or is an empirical issue?

A personal observation:  I have a very powerful solid state amplifier and a small tube amp  (over 1000W vs. 10W)  and I've recently discovered I prefer the sound of the big amp over the little one with my very efficient speakers (>99db).  I recognize that there are a lot of variables at play here.

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.

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.