I would think that you would not have a problem driving the speakers.
My amp does not have a switch or selection. I just hooked in the speakers to the back and it works at 5 ohms. I had 8 ohm speakers before.
I think the best speaker for you would depend on the size of your room and how you are going to place the speakers. I put my source in a smaller room yesterday to listen to the difference in sound. The room was more of a square shape and the speakers were too much for the size of the room. I could have put damping material up but the speakers sounded better naturally in the room that was longer and bigger. I really do not have to put sound treatments up in the bigger room. The smaller room would have required a lot more work to make them sound good.
I have never owned dipole speakers before and always owned regular floor standing speakers. It really does make a difference how the dipole speakers are setup in the room, since you have sound coming from the back and the front. It is also has its pluses since the sound coming out can have excellent presence and sound-stage. A lot of times, I am listening now to my music and find myself getting lost in the music. The sound of the instruments envelops the room, especially now that I am learning how to place them in the room.
The source are listed at 5 ohms. The MartinLogan preface are listed at 4 ohms. Your amp may be able to drive the Source easier. There is also only one 8 inch driver in the Source compared to 3 6.5 bass drivers in the Preface.
You may want to ask a question of people who own the Preface speaker about how sensitive they are to room setup. Personally, I think it is worth the problem of room setup for the Source, especially with the payoff of the sound reproduction, presence, and sound-stage.
I do not know if you are married but this may make a difference. My wife thinks the Source speakers are ugly. Personally, I really like the way they look. Your spouse may not like the look of the Source speakers. If it is in your own dedicated room, then it should not be a problem.