i used to own a E660 Savage Se, not to be mistaken for the E670 SE ... and i can compare it with a dual rectifier (you didn't mention which Boogie). Engl has some great cleans, maybe some of the best cleans i've heard coming out of a hi-gain head. the distortion is way different than the one from mesa's ... dual recs have 'a grid/sandpaper' (i don't know if you catch my drift here) in their tone, while Engl has a very compressed and 'clean' distortion ... it's a very usable distortion for different kinds of music, but to me it sounded a bit too faceless. the savage se used to have a 'crunch'-channel, but i didn't care much for it: it wasn't marshall like or whatever the promotion talk tells us it's supposed to be. it's more than decent, but ... you know.
i used to run the amp on 6l6's ... it was built like a tank and very reliable, but practically it also handled like a thank ... these things are huge, trying to get through a door with this thing is a challenge
the only thing that failed twice on me was the midi in the amp ... there's a lot of electronics going on in those amps and if something failes it can take a while for a tech to find out what the problem is. other than that i never had any probs with this amp ... owned it for 3 years.
the equalizers run different from mesa, simplified: mesa amps have interacting equalizers and engl have seperate equalizers ... which means that if you turn up the bass, it's only affecting the bass in your sound and not you whole sound. i found that the amp sounded better (hi-gain channel) if you gave it a kick in the *** with a lot of mids & bass and rolled back the trebs. also i noticed that she sounded better with the volume kicked to 'are you knuts'-levels and the gain rolled back to 3 or 4.
in the end i sold it because it was literally and figurly too big ... too much channels and options for my liking (42 knobs & switches, wtf) and huge and heavy to transport. it was a good amp though.
i hope this helped a bit.