My office setup

If I understand your description correctly, you're going wirelessly to an Airport Express and then using an analog stereo-mini to dual RCA cable to connect to the preamp, is that correct?

If that's it, the weak link is that you're using the internal digital to analog converter in the Airport Express. It would be much better to use an external DAC with a Toslink input, connecting to it with a digital Toslink cable from the same port on the Airport Express. Then the analog outputs from the DAC would go to your preamp.

Virtually any external DAC you can buy would be a big improvement over the tiny chip in the Airport Express. Finding a used DAC or one of the inexpensive imports on ebay would be a place to start.

The signal that's being sent from your Mac is a digital signal so it has to be converted to an analog signal somewhere along the way. The Airport Express will do it for you but it was not designed as a high-end audio component and it's much better to hand that task off to a device that is designed specifically for it.

Most DAC's do have a Toslink input, but not all, so check that before you buy something. The same stereo-mini outlet you're using now for analog output will also accept a Toslink cable with a mini-Toslink connector, or you can use a standard Toslink to mini-Toslink adapter on a standard Toslink cable. Most DAC's that accept Toslink will use a standard Toslink connector on that end. So, you'll need some combination of cable and/or adapter that gives you a standard Toslink connector on one end of the cable and a mini-Toslink on the other.

A search of the site will probably give you some good information about DAC's to investigate.

Jax2 is right, I should have been more thorough in my response. The sound of the analog output of the Airport Express isn't nearly as good as using the digital out to an external DAC but you shouldn't be getting anything I would describe as distortion. Isolating what's causing the distortion before you spend money on a DAC would be a smart move. I've