Depends on how much you are willing to spend on preamps. If you go out and buy yourself a battery of Art "Tube" MP's you're downgrading. If you have a couple grand to spend then you could probably improve the sound. I'm sure someone will argue that regardless you are still running through the FP's preamp circuitry as the line inputs do not bypass the preamps entirely but, I don't know enough about it to say.

Maybe I'm mixing up my PreSonus products here (entirely possible) but I recall looking in the block diagrams and manuals recently and seeing that the line inputs are in fact true line-ins and do not go through the mic preamp.

I might be wrong though...

Well, you could also be totally correct. I do recall the subject coming up before but I've never seen the schematics.