I just heard an Xotic BB and BB+ through a boutique amp (Nolatone Superbug?). The BB & BB+ sounded like EL34s to me the way it was breaking up. Anyway, it was a nice preamp or distortion pedal, however you want to use it. I suppose it would be nice (& expensive at $200) if you want to get an EL34 sound with a Mesa, but for a 6L6 sound, why not just use the Mesa's distortion? I never figure this one out except that we all like to buy a new pedal. It's like a man purse, isn't it. And like my wife's shoes, I never get ride of one!
If I go back to depending on pedals for overdrive, I'll just play my 5E3 or one of my old Fenders, & not deal with all the weight & complexity of a channel switching amp. If you just want a boost, seems like there are cheaper pedals out there. With my PRS, the pickups are so hot I couldn't imagine using a boost. With the single coils, some boost might be nice, but it's noisy enough as it is.
That being said, the Express 5:50 I bought really does take pedals well, both on input & in the loop.