OK to hook up both RCA and XLR on amp?

The amp needs to have a toggle switch to select one input or the other not both at one time. I have used this type of setup for over two years with a Classe' CA300. I have not found any difference in sonics or noise levels with the un-used input switched off versus removing the set of cables from the other input. I highly recommend this setup for it's flexibility.

You do need to contact the manufacturer on this to make sure you don't blow anything up. A few months ago, I contacted McCormack about this. They said it was fine since my modified DNA amplifier had a toggle switch to switch between XLR and rca. They did comment that there would be a slight reduction in sound quality. I could not detect this degredation. Some points on this:

1. If your amp does not have switchable XLR's, you will probably get a buzzing when you try to use the rca input while the XLR input is active (unswitched amps use a little gold shorting pin to turn off the XLR input). Switchable means there is actually a toggle switch that you flip to 'use rca' or 'use xlr'. If you don't have this switch, you probably are going to get the buzzing on the rca.

As a potential solution: there are some very good preamps on the market that have a 'processor input'. This means you can have a VARIABLE input such as the output of your home theater receiver, or even the variable output of some TV's feed your preamp. The adcom 750 is a great sounding preamp with this feature. This is a great way to go because you can use your TV/home theater volume control (esp in a multi-channel environment).

Another alternative might be to use a line level output from your receiver to your new preamp, then use the new preamps volume control (or program a learning tv remote). The disadvantages of this are that you A) would need to use your receiver's volume for the rear channels, and the preamps volume for the front channels, and B) have to juggle two remotes to watch TV.

I would like to emphasize something that Elizabeth said, contact the manufacturer, but add that unless the inputs on the amp are user selectable switched inputs, you might need to rethink this arrangement. The non-operating unit, either your receiver or pre-amp, and their associated ICs may affect the sound quality, introduce noise, or both. Additionally, you may want to contact the receiver and pre-amp manufacturer to determine the affect on their components output stage, i.e., not cause any damage when subjected to this configuration in a long-term situation.

Anyway, these are some of the concerns that I would have with this configuration. Hopefully some EE will weigh in and provide a more definitive solution.