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...