A very nice bloke loaned me a First Watt B4 which I've been trialing in my biamp system: 4343b's, Sansui au10000 on the Midbass HF and UHF, Perreaux 6000b on the woofers.
Crossovers I've previously tried: jbl m552, Behringer Super X pro, Ashly XR2001. The Jbl m552 was the worst I'd say, the Behringer slightly better, and the Ashly far better. So given how much of an improvement the Ashly made, I thought I should try something better again as I hadn't appreciated the effect the xover had.
So I setup the B4 with a simple Sallen-Key, 12db/oct, 300Hz crossover point. First 10 minutes of listening with no EQ I sort of thought "yeah it's maybe a tiny bit better then the Ashly". I don't know if it needed to warm up but after a while I started to hear some small details in a few familiar old tracks that I hadn't noticed before. (e.g. As Morrison is singing the words "riders on the storm" at about 6min point in the track, someone else is whispering the same words over top, giving a creepy vibe)
So then I EQ'd it with a Klark Teknik DN360 and REW and listened some more. The improvement over the Ashly was now a bit more obvious. The center of sound stage was really tightly focused, my setup seemed quieter (the Ashly must have been adding a bit of noise) The highs were a bit smoother and less fatiguing possibly too. The improvement was probably even a bit better when I then switched from Digital to Vinyl, as that's where the Ashly was noisiest, while the B4 was dead quite.
So yeah it's a step up from the Ashly IMO. It's a much more expensive unit though of course, but my experience is you'll just be unhappy with cheaper units and end up trying several as I did. I definitely wouldn't bother with the jbl m552 or the behringer, the Ashly is probably the best quality/price compromise but if you can go to the next level I think the B4 is worth it.
Cheers.