I personally find the Martin Logan and Vianna Acoustics to be quite different, although I'm not sure I've heard the ML Preface specifically.
My personal opinion is that the amp is going to make MUCH less of a difference in what you hear than the speakers will. I'd put the majority of my budget into the speakers.
Aperion have been said to sound similar to the VA's. They are an ID company that allow a risk free in home trial for 30 days, so you might want to check them out.
Salk Songtowers are also highly regarded for their detail and neutrality, but they are also an ID company. Perhaps you can find some owners in your area and go take a listen to either the Aperion or Salk, or both.
I don't think you'll get as good of sound out of $500 towers as you will $1500. Similar? Maybe, but definately not as good. The big thing you gain, IMO, in going to more expensive speakers is more detail and clarity. There are many other good choices in your price range, perhaps going out and listening to some more speakers would be the best thing at this point.