DBX is just a code name for something they are working on. This could be the Rapide, the AMV8 Vantage Vantage, or the replacement for the Vanquish.
Dr Bez has ruled out building a SUV, and Dave Richards agreed with this.
The DBS was just going to be a DB9 Special for the movie, but like the Rapide the demand from the proto and spy shots has prompted them to build production cars.
The 700bhp claims come from Dinian in Germany claiming they can now upgrade a Vanq S to around this.
The manual Vanqs are all post production modifications, and any car can be modified to this spec (both S and non-S).