Audio Woes

I've been podcasting for several years doing at times three shows. I have always experienced poor audio recordings from my mic/system.

Here is the setup:
Heil PR 40 mic going thru either a Symetrix 528e voice processor or a Golden Gate Pre 73 pre amp.

Signal then goes out to an Allen & Heath 24 channel mixer then via USB to my computer. I'm running Soundtrack Pro recording software on a 24" iMac.

You would think that the recorded audio would be pretty good with this setup. It isn't. It's noisy and in the broadcast sense, unusable. I've been trying to isolate the problem for over a year now with no luck. Should I go with a Mac Pro and use its soundcard instead?

If anyone would like to hear a sample file and give a professional opinion, I would be more than happy to send.

Thanks,
Mark