What are you referring to when you allude that the DSX does not support CVs 3 and 4? The technical reference for SoundTraxx decoders is a combined DSD/DSX document that applies to both types.
The DSX installation instructions that come with the decoder list no CVs at all, so you have to download the DSD/DSX technical reference manual from their web site and refer to it instead of relying on Decoder Pro to be right.
For some inexplicable reason Soundtraxx does not make clear that the DSX does indeed have CVs 3 and 4 that can be used in the normal fashion to ramp up motion (in this case sound of motion) These can be programmed in exactly the same way as any other decoder's CV 3 and 4 and together with CV 58 can provide satisfactory methods of matching ramp up of the statonary decoder's sound to the motion of the motored unit. (I have these in dedicated "sound cars" with 4 large oval speakers) and can get the sound and motion now quite realistic.
I don't know why this is not put in the documentation, but it has also escaped the notice of the JMRI designers who do not show the CV 3 and 4 in the DSX panel of Decoder Pro either.