Is it a no-no to plug the output of an external pre-amp into my Mackie Onyx mixer (routing through the on-board pre-amps) before going into Reaper? Why?

I guess I just never thought about this until now. I'm using the Onyx as a digital interface for Reaper, and up to now I've used the preamps that come on the mixer to boost my microphones (I record all acoustic). I want to try a higher-end preamp on one microphone but I'm still going to have to go through the Onyx to get to Reaper, so my first assumption was that I'd just come out of the external pre-amp into the same XLR, as if it were a microphone without a preamp. I figured as long as I didn't use much of the gain on the mixer's pre-amp, I'd be fine... and to be honest I still don't understand why I wouldn't be... but, I hear you on the "line-ins" for each channel. I guess that's a way to essentially bypass the mixer preamps. I don't have an external preamp to try this with yet, and at the end of the day all I'm looking for is a good clean boosted signal (not much coloration and certainly no "effects"). I'm in the process of reviewing my preamp options and wanted to understand how I was going to hook all this up before I buy something.

Thanks guys. You know, I think I was also looking to cheat the system.... My mixer preamp is cranked nearly all the way if I'm recording with a SM57 or other dynamic mic... It's not horrible, but I want to cut the noise a little. I figured If I could boost the signal at 30-40db with an external preamp, then hit it with another 20 db from the mixer preamp, I'd have a very high boost, with low noise. But I guess it doesn't really work that way, or that's what everyone would do. Thanks again for responding. -fm

onyx with the outboard pres + onyx drivers 

No trouble there. I do this all the time with my onyx 1640 (older model) I have several nice outboard pres which I will feed into the 1/4" line in. I've done a lot of recording this way and I get good sounds. BUT, you still need to go through the gain stage in onyx. Even when you go line in, the schems show that you are still going through the onyx gain, not bypassing it. I even tried to go through the inserts, by rigging up a crazy adapter to go into the insert, but not send back out. It was without success. So, as these folks are saying, just be careful about your gain staging and you'll be fine. Work downstream from your source and watch your meters all the way...then at onyx just bring up your gain enough to hit 2 or 3 LEDs. Solid Gold. Btw: I'm just transitioning into Reaper and I'm very excited about it. I'm curious if any other onyx owners have any issue with the new onyx driver and Reaper? I have a brand new machine and win64- but the onyx driver seems to be a little unstable/buggy to me...