ESLs traditionally are a tricky problem for solid state amps. The reason is that solid state amps are often able to act like a voltage source, which is to say that they can often double power as the load impedance is cut in half.
This causes troubles with ESLs, as unlike cone-based speakers with a box or baffle, the impedance curve of the speaker is not an efficiency curve as well. IOW, the speaker has the same efficiency at all frequencies. Additionally, the typical ESL has an impedance curve that varies about 10:1 if it is a full-range system. For example, Sound Labs vary from a high of about 30 ohms in the bass down to about 1.5-3 ohms depending on the position of the Brilliance control. The Quad ESL57 varies about 45 ohms in the bass to about 4 ohms in the treble.
So the ability to double power as impedance is cut in half does not help so much- you get too much high frequency energy. This is why tube amps are usually the preferred amp if you run ESLs.
However Martin Logan has understood for a long time that solid state amps dominate the market, so they have kept their impedances low in order to limit the amp's ability to double power. As a result they are often only 0.5 ohms at 20KHz! To drive this with a tube amp a set of ZEROs is a good idea
http://www.zeroimpedance.comThe more feedback the tube amp has, its likely it will also sound brighter for two reasons. First, most applications of negative feedback contribute to brightness as it causes the amp to make higher ordered harmonics (while overall dropping the THD) which the ear/brain system uses to detect loudness (which is another way of saying that our ears are very sensitive to higher ordered harmonic content; much more so than human vocal frequencies). The second reason is that the more feedback is used, the more the tube amp will try to behave as a voltage source, which does not work so well with ESLs as previously explained.
More:
http://www.atma-sphere.com/Resources/Paradigms_in_Amplifier_Design.phpSo- I would get the biggest tube amp you can that does not run negative feedback, and get a set of ZEROs so the amp can handle the otherwise difficult load. This will get you the speed and delicacy ESLs are known for, but without excess brightness.