Well I would have to say that the Quad and the studio are the best preamps Boogie ever put out. I also own 2 Triaxis preamps that I never really use. The Triaxis is not capable of putting out a super high gain sound without becoming muddy, but they are great for blues stuff.
You are much better off with the Engl. It is better than any Boogie ever made.
Boogie amps have become such garbage in the last few years since they started mass producing the cookie cutter style amps.
The trouble with a Quad or Studio, or any Boogie for that matter is that you have to find a "GOOD" one.
Before you buy one, definately try it out.
In my experience 75-80% of Boogie amps sound very muddy with no bottom end definition, and a buzzsaw like cheesy distortion like a rectifier.
You would be happier with a JMP1