What is the best premp for Atma-Sphere MA-1 3.3 amp?

@OP, 
I PM'd you, but didn't indicate the build model.
I own the 3.3 version MP-3.
Also, when I called Atma to inquire about upgrading the MP-3, he said the 3.3 was an entirely different build from earlier models, and to upgrade an earlier model would cost a significant sum. You can see the prices on Atma-Sphere's website.
BTW, one of the reasons you find so many satisfied Atma customers is that you are only a phone call away from help, should you encounter an issue. Most likely it will be Ralph answering the phone, and most likely it won't be the Atma causing the problem- believe me, I know.
Bob

@OP, 
From a quick scan -I didn't see which model of  Sound Lab speakers you own. It would help determine you power needs.
Bob

@OP
I just did a quick google on the m545's ( I don't know which level you are using- Audiophile/Majestic/Ultimate, not that it seems to make a difference in power requirements). All seem to be 50-500 wpc.
So, the MA-1 should have plenty of power- unless you want rock concert levels.
The Sanders and Bricasti seem to be SS amps. Are you interested in SS?
Bob

@OP,
In my limited understanding of such things, amplifier power ratings are for ballpark estimations of a speakers demands/capabilities.
The real issue is how well an amp can handle transient power demands. Which is why I lean toward having an amp leaning toward the higher power ratings.
Your speakers are rated for 50-500 wpc. Both the Pass and Atma amps fall nicely in the middle. And, given how Mr. Pass tends to underrate his amps, I bet they can put out a bit more power without issue of clipping or distortion.
Now, the reality check:
Unless you are really cranking up the volume, I doubt you will be using more than a few watts with normal listening.

So, why get an amp with 140 or 350 wpc?
Like I said, it is the ability to handle the transients within the music, and not taxing the amplifier to the point where it introduces distortion.
Like I said, I am no expert, so there will probably be some disagreement.
Perhaps Almarg can provide a better description. He knows and can write a very clear explanation, if anyone can.
Bob