I'd go with the speakers. I'm using the MAXX-2's with a VTL 5.5, which may be a step below your ARC and the sound is fantastic. I passed on the WATT/Puppys because, when I compared the two, the WATT's seemed a little bright, the height of the soundstage was limited, and they did not play loud. The MAXX's, to my ears, are much more natural sounding. I'm sure your ARC is up to the MAXX's and will yield a much bigger improvement over upgrading the pre.
Good luck!
Good luck!