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.

Thanks guys. You are right Sfar, I am going wirelessly to my Airport Express. I didn't know that it had a internal DAC. I will look into getting an external DAC. Do most come with this Toslink input that you're referring to?

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.

I'd completely agree with Sfar regarding making an improvement in the digital conversion (the DAC in the AE leaves a whole lot to be desired). But if you are hearing "distortions" I wonder if it isn't the amp(s) in those active speakers? Why are you using a preamp? Have you tried removing the preamp and going straight to the speakers (I'm assuming they have a volume control)? Usually active speakers do not require a preamp in between to boost the signal. Regardless, an external DAC will likely improve things, though I'm not certain it will get rid of the "distortions" you are hearing - can you expand on that description?