The new DX series (and the impending Analogue AX series) are much more flexible than the existing D and A series products.
The new devices run on the same OS as the powerful CMS and Call Relay SP products and therefore offer a much higher level of functionality including CODEC auto-negotiation, the ability to set up multiple routing groups and dial plans, and SIP user agent.
Also, the DX series is available in MultiPath mode (the same as the D3000 where calls can be failed over to PSTN) or in Gateway mode. In Gateway mode there is no fail back to PSTN but this is rarely used in a carrier environment and brings the cost per port down dramatically.
It would appear to be an intelligent move by Quintum to ensure that the whole range of products will be running a common, powerful, OS.