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.
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.