I had the SL3's and liked them very much. A step up from there would be either a pair of Vandersteen 2's or 3's. They are just as coherent as your panels but offer more resolution and body. I highly recommend you listen to them first, though. They're not as forgiving as your SL-3's. If you listen to a lot of bad recordings, its something to keep in mind. If you can find a good pair of ProAc 2.5's, you will probably like them as well. They fall somewhere in between the ML's and the Vandersteens.
Replacing my Martin Logan SL3 speakers.
I am thinking about replacing my Martin Logan SL3 speakers with box speakers, and I am looking for suggestions. I mostly listen to classic rock, but also some jazz and classical. My budget is around $5000 new or used. The rest of my system is
Audio Research Ref 110
Audio Research LS 25mk2
Audio Research Ref CD7
Clear Audio Innovation Wood Compact w/ Universal Arm