You're right, this would be a difficult one to answer, as you already have excellent pieces in both places. I will stick just to the items and brands you mention; it's obvious that you like the sound of your system right now (expect the Wilson-bashers to suggest some other speakers!). The safest bet, assuming you could stand their looks and have a room that can handle the extra bass, would be to change the speakers. From my hearing them and others' posts here, the Maxx 2 is a significant upgrade over the WP7; and while I think the preamp upgrade is likely to be an even bigger improvement, you risk upsetting your system's synergy with the difference in "sound" between the two preamps more than you would changing the speakers, which have a similar "house sound". I will say that you would certainly hear the difference in preamps through the WP7s, but the difference may turn you to changing other components in the chain. If you really have the bug, borrow the preamp from your dealer and hear it in your system for a few days; if you don't miss the tubes in the preamp then you should go for the new preamp. Just my opinion, how I would go about it.
Upgrade order suggestion preamp vs speakers
Potentially, a difficult to answer question but was just curious how the audience would respond. What would be the most beneficial upgrade for the ears: preamp (ARC LS25 mk II -> boulder) or speakers (watt/puppy 7 -> maxx2)?
Ultimately, both components will likely be upgraded but I may live with this upgrade for a few years.
Ultimately, both components will likely be upgraded but I may live with this upgrade for a few years.