I used both my Studio and Quad into the fx return of my Tremoverb head for quite some time with great results. The tone is pretty much what you would expect from the Mark series but it doesn't really have any of that Recto thumpy low end. Still heavy and cool, just different though.
My racktifier just got delivered today but I can't check it out until I get home on Sunday, so I'm anxious to see how it sounds as well as how the preamps sound through it.
mj