?Best speakers for a 2000 sq.ft loft?

The original B&W800's, preferably actively bi-amped. This speaker is truly a bargain, even at its original price, and a downright steal on the used market. It can be first run normally and as finances permit, bi-amp actively.
This speaker can produce 120 db at 4 meters easily (with the delicacy you are use to with the M-L's.)
It will fill your room and then some.
Richard

If you want to stay with Martin Logan, a pair of Prodigy can fill up a 35'x60' room very nicely.

I saw an ad for the Genesis 201's here yesterday for less than half price. These are 4 towers w/multiple driver arrays. Two of the towers are bass/subwoofers, and the other two are tweeters and midrange. this is an incredibly good-sounding system, it blew me away, though I ended up with the Genesis 500's because I didn't have near enough room for the 201 system. If you've got $10,000, you should try to hear them.

the new b&w 800 seires will work well because they love an open area as such. Good Luck Gino

Your M-Ls should work fine as long as you sit in the near field. Place the speakers in the free field (a good thing) and seat yourself near a wall (for bass reinforcement). There's no need to pressurize the entire room.

anything really huge Pipedreams, Genesis 1's or similar, that's alot of air to be moving your going to need alot of power also suggest VTL Wotan 1200 amplifiers.

I agree, your SL3's will get lost. At the kind of distances involved in a 2000 square foot room, hybrids are out. This is because the panel will approximate a line souce while the woofer acts as a point source, and sound pressure level will fall off more rapidly with distance for the woofer than for the panel. So the SL3's will sound thin and weak in that room.

I have no idea what your budget is, and unfortunately that's a significant part of the equation. Also, what are the acoustics like - is the room echo-y? I'd hate to think what it would cost to acoustically treat a room that size, so we'd need a speaker that would work well with what you've got.

The ideal speaker would have good timbre, high efficiency, and a well-controlled radiation pattern. That big room could end up being a marvellous blessing or a hideous curse, and it will largely depend on the choice of speakers.

Also, can you describe the place in more detail? I'm not entirely sure that what I picture in my mind when I hear the word "loft" is necessarily what you intend.

I realize I haven't answered your question at all. Sorry, but I don't have enough information to get started yet.

Best wishes!

Whatever you get they will obviously need to be on the high-side of sensitivity. Consider horns, the Avantgardes perhaps?