If you want to use the HD500x for bass, the HD Bass pack is pretty essential. As you say, there's the B15 flip top on the standard model pack, but there's also the Fender Bassman & 4x10 cab that's worth a try, and I've had some good results using the HiWatt amp & 4x12 cab model. However, though it "only" adds 3 amp models (2 of which are variations of the SVT), it also adds the 8x10 & another 4x10 cab model, and the SVT Normal model is simply superb IMO.
If you run the HD into your amp's Effects Return socket (bypassing the amp's preamp), you'll get the sound from the HD without too much of the amp's colouration. I run mine into a MAG 300 2x10 combo, into the Effects Return, and it works for me.