This seems like an easy one. Both are tube amps but the Project appeared to have been designed for accuracy, so if you only need a pre-amp and transparency is your priority then it is the one to keep.
The Quad one would likely have a sound signature that if you happen to like it, then you should have returned the Project instead, even if you only use it as a pre-amp. All based on specs and their website description, as I have no experience with either.