Upgrade order suggestion preamp vs speakers

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.

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!

A little more information would be helpful. What amplifier are you currently running? What do you dislike most about the sound you currently have?

This has to be a no brainer, are you kidding me? ANYONE who remotely thinks that the preamp upgrade is in the same league with the speaker upgrade is smoking something. The Boulder preamp may be twice as good (heck 10 times as good) as the ARC, but 10 times better than something that is already down in the weeds, so to speak, isn't going to buy you much.

The speakers may be only 50% better, but that's where the rubber meets the road. A 50% speaker improvement is HUGE and will make you forget a preamp upgrade. At the very least, if and when you do try another preamp, you will know the utility of the change in your final system configuration.

It's ALL about the speakers! Don't think so? Try a set of headphones (good ones) and see how great your system sounds. I went from Dynaudio Confidence to Evidence Temptation and the change was UNBELIEVABLE. Waste your money on the preamp if you must, but changing out your speakers is the ultimater easy choice.

Beneficial change? You got to be kidding! Ditto Bigbucks5. Get out your wallet.