have you tried different tubes for the sachs preamp?
most 6SN7s can be overly smooth.
This statement is false. Design can have an enormous effect, but 6SN7s of themselves are quite linear.
I have Don Sachs 2 tube preamp driving Pass Labs X350.5 driving Sound Lab M545 ESL speakers.
The Sound Lab ESLs do not expect that the amplifier will behave as a voltage source. To deal with this fact, the Sound Lab has adjustments on the backplate to allow it to compensate for different amplifier types. So you have a wide range of amplifiers you can use.
They are best driven by tube amplifiers. The reason for this is that tube amps generally do not double power as impedance is cut in half, and are less perturbed by the 30 Ohm peak in the bass region (put another way, they don't have their power cut in half as impedance is doubled). ESLs generally have an impedance curve that varies by about 10:1 from the bass range to 20KHz and the Sound Lab is no exception. But the nature of the driver itself is that it needs the same power at any given frequency (and thus any impedance) to make the same output. That is the nature of ESLs. So they are not 'voltage driven' as so many box speakers are.
That is why the Sound Lab has so many adjustments on the back. You'll find that to really make the speaker play with transistors, you'll need about 600 watts, since into 30 Ohms a solid state amp will make about 1/4 of its 8 Ohms rated power (and thus is really about 150 watts on that speaker). But a tube amplifier might make considerably more power into that impedance. For example, our MA-1 power amp makes about 150 watts into 30 Ohms and about the same power into 8 Ohms. So you can see how an amp like that can keep up with a 600 watt solid state amp on this speaker.
The best Sound Lab installations I've seen and heard have used Purist Audio Design cables. We've shown with them a lot at audio shows and that tends to be the cable of choice. You've not mentioned the dimensions of your room, but I am assuming that its of average size since you didn't mention any power restrictions while using the Pass Labs amps, which are a bit underpowered for this speaker (although as solid state amps go, is one of the better sounding ones out there; if it comes down to your use of a solid state amp, if they have enough power for you I would **not** change them out!).
But you do want to be sure that there is at least 5 feet between the rear of the speakers and the wall behind them. In this way the reflected information bouncing off of that wall can be used by the ear as soundstage information and will make the imaging more palpable. If the speaker is less than that 5 foot minimum, the delay will be too short and the ear will interpret that as harshness!
If your speakers are properly set up in this regard and you don't want to go with a tube amplifier, then I would be looking upstream to get greater resolution. The Pass Labs amps really are some of the best solid state amps you could lay your hands on.
