The pres in the Quartet are fairly netural, but that's hardly a bad thing. They work quite well on most sources I've thrown at them (bass, drums, vocals, horns). While they won't ever be character preamps, they have more than enough gain for ribbons mics and gain-hungry dynamics (think SM7b). As for the conversion, I owned a previous-generation Ensemble and was pleased to hear tighter bass response in particular with the Quartet.
I have not used the Clarett series, but I owned an ISA 2, and those preamps had a certain depth and dimension to them that were beyond the Quartet's on-board preamps. They added a sparkle to overheads and acoustic guitars and stacked extremely well. The Quartet preamps stack well, too, but this is something I felt the ISA two did notably well. It may be a bit unfair to compare to them to the Quartet's on-board preamps, but that's what I've got experience with.