I think Dr Tone is correct, based on the specifications there is no difference between the Pro and Expert USB boards. The difference I noted, and I have to be clear that this is my subjective opinion, is that comparing the microRendu via USB versus the microRendu into Mutec via AES/EBU, there was a definite improvement in bass accuracy (accuracy, not quantity) via the microRendu/Mutec with AES/EBU. Beyond this specific point of bass accuracy I cannot say I have noticed any other differences between the Pro and Expert USB, they might be there, but I cannot say I have observed anything. In the Pro I think the difference with bass accuracy is significantly smaller, with the Pro USB seemingly improved in this respect. My supposition is that this is maybe related to leakage loops / isolation. So given the USB board is the same, maybe in the Pro somehow the galvanic isolation is a little improved? This is of course a tricky thing to observe. The USB input of the Pro will sound better than the USB input of the Expert because the amp sounds better, you cannot separate the two. So it becomes a subjective observation of one specific source via USB versus another via AIR, AES/EBU or S/PDIF. Plus, as most of us have never had their Experts and Pro's at the same time, it does rely on memory of the differences. (although I do note that Rama has a 'spare' D200 gathering dust if he ever fancied a weekend of A/B testing 
) Another subjective observation I have with the old Expert is that the very best thing I heard through an Expert USB with respect to bass accuracy was the CAD CAT music server. When I listened to the CAD CAT it was demoed with an early version of CAD's 'ground control' device, but this was connected to the CAD CAT, not the Devialet, so I remain puzzled why the CAD CAT was better than an mR, Melco or whatever in this regard. Maybe some other factor completely, indeed, there are a lot of factors working together so these things are difficult to pin down. It would be nice to have some input from Devialet in this regard, but not much chance of that unless they let Mathieu Pernod out to play at a dealer event some time.
Getting this back on topic, I have to say my observations do concur with Hifi_swlon's, in that even with the Pro USB, the bass accuracy via AIR is a touch better than with a micrRendu via USB. Or to put this another way, in this regard AIR is a lot closer to the performance of a Mutec via AES/EBU than a microRendu via USB. (Normal caveats, my system, my ears etc.)
) Another subjective observation I have with the old Expert is that the very best thing I heard through an Expert USB with respect to bass accuracy was the CAD CAT music server. When I listened to the CAD CAT it was demoed with an early version of CAD's 'ground control' device, but this was connected to the CAD CAT, not the Devialet, so I remain puzzled why the CAD CAT was better than an mR, Melco or whatever in this regard. Maybe some other factor completely, indeed, there are a lot of factors working together so these things are difficult to pin down. It would be nice to have some input from Devialet in this regard, but not much chance of that unless they let Mathieu Pernod out to play at a dealer event some time.Getting this back on topic, I have to say my observations do concur with Hifi_swlon's, in that even with the Pro USB, the bass accuracy via AIR is a touch better than with a micrRendu via USB. Or to put this another way, in this regard AIR is a lot closer to the performance of a Mutec via AES/EBU than a microRendu via USB. (Normal caveats, my system, my ears etc.)
1000 Pro - KEF Blade - iFi Zen Stream - Mutec REF10 - MC3+USB - Pro-Ject Signature 12
