This was primarily because of the change from a 16 bit DAC to an 18 bit DAC but their better than average utility and software bundle did not hurt the cause. Believe it or not, Voyetra Turtle Beach were actually the first to ship a Vortex 2 based product. The OEM 2 speaker Montego II was shipping in Dell machines in September of 1998, ahead of Diamond's launch of the Monster Sound MX300 then just days before the MX300 hit the streets, Voyetra Turtle Beach decided to offer the same board that they were shipping to Dell from their own phone and web based storefront as the Montego II Whitebox edition. Diamond, being first out of the block and having a very strong retail presence seems to have a clear advantage in the North American market. In the UK and Germany, I believe that both Videologic and Terratec have a very strong presence which probably negates any disadvantages they may have encountered by shipping well after Diamond.
Lets have a look.
As I noted above, the Turtle Beach Montego II Quadzilla is powered by Aureal's Vortex 2 sound chip. The card itself offers the following features which are flow from the reference design and the Vortex 2 chip. 76 3D streams for DS3D and A3D 1.x are accessible in a future driver upgrade from Voyetra Turtle Beach. The first and most obvious variation from the reference design is the decision to go with a main card with 2 speaker support and follow later with a bracket board to add additional functionality, including four speaker support. It allows for modularity.