Been doing some research around this for a while. I've been split as to whether to upgrade my amp to a more modern, Class D amp, new cab / new speakers, dedicated bass combo, or an FRFR set-up. For the time being, I decided I'm going to stick with my amp & cab, but just use the Effects Return socketinstead of plugging my pre-amp setup into the amp's pre-amp. It does work, and sounds pretty good too.
I'm currently using a Line6 POD HD500x. I've owned a B3, an MS60B, a PODxt, and a PODxt Live. I liked the B3, but didn't like the fact that after using the amp sim, I only had 2 spaces left, add in a Compressor or EQ, that left 1 space for another effect. Pretty limiting. Never really used the MS60B, bought as a backup for the B3. I
already had a bean PODxt, so I bought a PODxt Live, more scope for effects, sound was slightly better than the B3 (despite being an older device), but to my ears, the effects were a bit too subtle, so I sold it.
Bought the HD500x, very happy with it, sounds great, and you can get some great deals on them now that everyone seems to be upgrading to Helix.