Speaker Pairing - First Watt F8

I would think Cornwall IV’s. As I’m reading your post I’m checking off boxes with each sentence. I’ve had very good results with lower power Pass amp, XA25 and Cornwall’s I think they are a fantastic combination, so much so I was thinking of getting an F8 myself as I regrettably let my XA25 go a couple years ago. 

I think it depends a lot on what you're looking for in the sound characteristics and what you're listening patterns are. My experience is that horns, like Klipsch, can sound amazing and have a very wide sweet spot but that they don't image as well as a standard driver speaker. The "liveness" is very appealing and fun to listen to. I have a dream of a second syst that's SET and horns. For me though, I love a detailed soundstage and while I definitely lean musical in sound, I also like a fair bit of detail. For all these reasons I'm not a horn guy but then that leaves you in a difficult space because you have quite a room to fill. I know the Devore's are definitely designed to run with low wattage but I'm not sure you'll fill that giant room. 

I'd encourage you to take a different approach. I always learned that you build a system around the speakers. It's clear that you love your first watt amp but if argue what you love is it with your current Zu speakers. If you want to meaningfully upgrade your system find the pairing speakers you absolutely love and then go from there. Don't limit yourself to what you can drive with your first watt. If needed, sell that too and get something that can drive the speakers you love. 

I have Vandersteen Quatros. I heard them and fell in love with them. Over the 8 or so years I've had them I've rebuilt my entire system around them. Every time I make an upgrade I just hear more of what I love about my speakers. I'm aware Im not answering your question, just providing an alternate approach. 

rmdmoore makes some great points.

I have a pair of Cornwalls in a room about the size of yours with similar intent. They are big speakers! And they do a terrific job of filling that room with energy and wonderful sound.

A couple of weeks ago, I swapped out the Cornwalls for O/93s. The Devores sounded great, but they lacked the...jump of the Cornwalls. I'm not talking about volume. I like the sensation of being "in" the sound, and the Cornwalls do that better in this bigger room, a room in which I have music playing a great deal of the time, not because I'm doing "serious listening," but because I like it.

The O/93s are fantastic speakers. They have detail that the Cornwalls just don't (for me), and I love nothing better than plopping myself down in front of them.

I like a lot of the stuff you like. I'm Zu-curious, particularly for the Soul 6. Volti looks very interesting. I think if you've got room for a big speaker, I'd look at a Cornwall or something similar (Volti?). This opinion is worth what you paid for it. And I never mentioned my low-level envy for your F8.

I guess I have to ask this question. "are you working to a better sound or do you just want to try new speakers?" 

+1 Volti Razz. Sounded dynamic and transparent at 2 different audio shows.