That is a nice setup but I prefer the Ref 5se over the GSPre. That said how much of the sound you heard can you attribute to the pre and how much the amp? Also the Vandersteen speakers are very different than your Magnepan speakers. You really need to hear the ARC preamp in your system.
Don't get me wrong, I own a Ref 5se. It spent over a week in my system before I bought it. I also had an LS27 in my system for a couple of weeks as well as a few other preamps. You can try borrowing the LS17SE. At least that will give you an idea of how an ARC preamp would sound in tour system. To be honest I did not much like the LS27 and would choose either the Ref 2 mk2 or Ref 3 over it.