Would switching amps make a big difference?

I have a pair of ML Montis. The impedance curve is quite complex, going from c.3k real at DC, to capacitive, to inductive at the crossover (c. 340Hz) to capacitive, to entirely real at c. 20kHz  (0.9ohms) to inductive.
Over most of the audio frequency range the impedance "waves" around 4 ohms.
I use several amplifiers to drive these speakers, including a pair of Rogue M180s, a Hypex Ncore400 and a Benchmark AHB2.
 All three amps are stable into the load- all reproduce decent 1kHz square wave outputs.The Rogues, used in the 4 ohm tap mode, have an output impedance looks that like about 0.3 ohms in series with an inductor, and there is a measurable and simulatable loss at 20kHz, but it only amounts to about 3dB, although very low impedance speaker interconnects are advisable. With the Benchmark the HF loss is substantially less, but surprisingly little difference can be heard between the two due to the altered frequency response.
The Ncore amp sounds rolled off in comparison to the other two, but doesn't measure significantly differently, which is surprising.
Neither the Benchmark nor the Ncore produce any audible high order harmonics in the audio band. The Rogues, on the other hand DO produce audible low order harmonics. The Rogues can be played subjectively louder than the Benchmark despite their having nearly identical power ratings into 4 ohms (180W vs 190W) with the Rogues producing several orders of magnitude higher distortion at the rated outputs.
Overall I have settled on the Benchmark as my amp of choice with the MLs.

I have the ML 13a's and was using a Prima Luna pre with Macintosh amp.  It was nice but I always felt something was missing.  Went to a Don Sachs all in pre and a pair of Odyssey SS all in monos.  I told both builders what I was using and they tweaked the builds to my speakers.  I am presently listening to the finest system I've had.....and I've had plenty.

It worked for me..
This year I swapped out a pair of Vincent Monoblocks for a pair of new PS Audio M700 Digital Monoblocks.. Listed in the best Amps in Stereophile this past year..
Big improvement and now a lot of raw power at 4 ohms, 700 Watts a side..
Better in every way detail, bass and soundstage..

Martin Logan ESL's all dive down to around 1ohm with -phase shift as well in the HF.
As  wynpalmer above found of the three he listened too, the amp best suited was the one that delivers without sounding rolled off or distant in the hf better into that load, the Benchmark, which can give considerably higher current's with lower distortion into those low impedance's. 

Cheers George

I would try a pair of Merrill Audio Veritas with your tube preamp and then buy the 11a's or better yet the 13a's down the road.