Check out the drum recordings on the web page below, which used an RME UFX with external preamps, and only 4 or 5 mics. I've used an older RME Fireface in tandem with a Lynx Aurora 16 for the last few years and like them more or less equally (different tones for different projects), but now I think (as a convertor) the UFX sounds better than both. Haven't tried the UFX mic pres, and never bothered with the Audiofire series myself, because of what I read about their headroom and noise shaping. Media: Audio Samples from AwTAC
According to the Audiofire 8 manual, yes, you are bypassing the mic input circuitry on all 8 channels (incl. 1&2) when you use the 1/4" TRS inputs. The rest of the preamp circuitry w/gain trim is still in the signal path, however, and from what I understand these circuits aren't optimized for high-level transformer balanced preamps the way a more professional dedicated A/D would be. Granted, you're likely to get more mileage from the outboard preamps you already have if upgrading the interface. RME only has a dB more headroom going in, but it's line inputs bypass the preamp circuitry completely, and really sound quite good.