I noticed that many mixers such as the Behringer hard wire their line inputs (without even a switch) in parallel with the mic input to the mic preamps. Yes, they do have a drop resistor but that's all. My concern is that the signal is being dropped and then amplified back up - thus adding another stage and more noise. No switch also means both a mic and a line source can be both "on" at the same time. I was thinking of adding a small switch and routing the line input to just after the mic preamp. Here I am just looking for reactions and suggestions.
A lot of classic, vintage, revered mic preamps have the same setup.
The added noise is negligible.
Two points here:
1) Most pre-amps that route the (attenuated) TRS input through the same pre-amp front end as the XLR input use breaking contacts (normalling) on the TRS jack socket. These disconnect the signals from the XLR when a plug is inserted in the TRS socket.
2) The thing that most serious audio folks are concerned about with the "everything through the first stage" configuration is not usually noise but colouration - the pre-amp affects the audio quality of line-level signals that may have gone through pre-amps of their own a few steps before. This can get bizarre; I've seen designs proudly boasting transformer inputs, but where the TRS input signals have to be brought right down to microphone levels before being put through the transformer, presumably in order to avoid core saturation.