I use a HD500X all the time for bass (rarely play guitar these days!), and I love it. The sound I was always after is right there, add in all the effects. Ideal.
It would depend on what amp model you use, and what sort of sound you're after of course, but for me, I found a great sound with minimal tweaking, then went from there (SVT Normal model & the 8x10 cab model).
I would suggest trying it again, and start from basics as you would a new amp. Set everything at 12 o'clock and work from there.
IMO, it is a great and very underrated unit for bass. It's a shame there are only a handful of amp & cab models available, but they're all usable.