PROBLEM with 24/96 bit-rate USB DAC that worked fine until I plugged in two

Question

I'm using a late 2008 15" Macbook Pro that I bought Jan 2009 right before the SD card models came out. Mac OSX 10.6.4 and iTunes 9.2 are installed. I have a Nuforce Icon HDP and a CEntrance DACport 24/96 bit-rate USB DAC (external sound cards) that still work great right now. Note - 16 bit/44 Khz is CD quality, and these 24/96 files are hi-resolution downloads that are closer to SACD or DVD audio.
I have been evaluating a new third 24/96 USB DAC plugged directly into the Macbook USB port (no hub) which has been working fine for over 2 weeks with 24/96 bit-rate lossless music files in iTunes. Audio Midi Setup is set for 24/96 to match the music file's resolution. After making suggestions to the manufacturer they made some improvements and sent me a new version to listen for the differences.
I listened to the new one for a day and it worked fine, but when I plugged in the original test sample into the second USB port at the same time as the new one in order to compare them, the sound became garbled and noisy. This never happened when either one was plugged in by itself. Note, if I make changes to the DAC's bit-rate in Audio Midi Setup I usually close iTunes first and then re-open it when I am done.
Upon hearing this new problem I unplugged both of the new USB DACs and plugged just one of them back in, but now it causes iTunes music playback to pause when one is plugged in and set for 24/96 in audio midi setup. Prior to plugging in both at the same time, either one worked just fine by itself. There is no reason when unplugging one or the other that they shouldn't start working again. But now I can't play any 24/96 music through either one anymore. In quicktime player the songs don't pause like with iTunes, but there is no sound coming through even with just one DAC plugged in at a time. Then I discovered that if I go into Audio Midi Setup application and lower the bit rate to 24/48 then iTunes will resume playing again and I can hear the music fine with iTunes or Quicktime player.
Basically, my Macbook Pro has now lost the ability to play 24/96 bit rate with either of these new USB DACs, when they both worked fine before I plugged them in at the same time. More confusing is that they use the same USB receiver chip as in one of the DACs that I mentioned above in the first paragraph, and both the HDP and DACport still work fine at 24/96 bit-rate. It's only these two new USB DAC that require me to lower the bit rate in Audio Midi Setup to 24/48 in order to listen through them, and this wasn't a problem as recently as yesterday!
The computer is acting like some setting needs to be wiped out - it's DEFINITELY NOT the USB DAC's that are faulty because they still both work fine at 24/96 on my wife's iMac (I'm afraid to try plugging in both at the same time to the iMac in case I screw that Mac up too).
I have reset the PRAM a few times, and this doesn't fix the problem. I thought maybe the com.apple.audiomidisetup.plist file could be corrupted and deleted it, but that didn't help. I don't know what to try next to get the Macbook to start fresh when trying to use these devices, short of having to re-install OSX.
Please, does anyone have a suggestion of how I can make the Macbook forget it ever saw these two USB DACs, and let me start fresh so they start working normally again? I do dozens of reviews for Head-fi.org and I can't afford to have my Macbook Pro compromised like this, which could prevent me from doing some reviews or evaluating new products.

Answer 1

UPDATE:
As noted in the first post, both of these newer USB DAC work fine at 24/96 on my wife's iMac, so it's clear that they didn't both become defective when I connected them to the Macbook Pro at the same time.
Resetting the Macbook Pro PRAM didn't help, so I removed the hard shell laptop case to get to the battery and remove it, then reset the SMC. After that I could use the original DAC at 24/96 again, but only IF it was plugged into the rear USB port. If I plugged it into the front USB port then the sound was garbled and filled with noise. This was a vast improvement over no sound at all.
Then I switched to the replacement/updated DAC, but I got no sound at all from either USB port at any bit-rate. And returning to the original DAC again now results in no sound again.
Sigh... I will reset the PRAM and SMC again, and see how far I can get with this. I'd still love some help. Thanks!