I AB'd my Focus and a WT-400 the other night. I love my Focus but I am saving my pennies to add a WT-550 to my arsenal...and that's alot of pennies, so I'd better start now.
The Focus has a wonderful, crystal clear tone. But it really only has ONE sound which can be tweaked to taste. I've experimened with different outboard gear into the Focus, but I have never found anything as good as the sound you get from just going into Focus' preamp. That said, the Focus is the bomb for doubling on BG/DB. It is really voiced perfectly for upright, and it also seems to work some mojo on fretless BG's. It's just a great, flat sound. The weight and headroom is also tops.
The WT-400 (haven't got my hands on a 550...yet) is mucho more flexible and gives up a nice warm coloration. The attraction to me is the ability to dial in a 3-D low midrange that will hit you in the stomach a little bit...something I've not been able to coax out of the Focus. I really loved playing with the parametric EQ and am starting to see the wisdom in having one.
To me, having both makes sense...especially if you're a doubler.