I have tried all of the above with similar results. I bought the DT50 212 after playing with it for an hour at GC with no effects. Felt this amp could be my next step up (with proper effects unit) from my Vetta II combo (long-time user, bought Vetta new). Especially liked the DT50 tube-like feeling, sound, and particularly headroom.. a weakness I felt of the Vetta II combo. I purchased DT50, brought it home and connected my G-Force in the mono effects loop. A bit noisy a first, but kept working with it until I was liking what I heard. I had resorted to G-Force post effects in the Vetta using SPDIF a couple of years ago, Vetta provided the pre effect (mainly compression), again a step up I felt. In using the G-Force with the DT50 I still had two issues - 1) no pre-preamp effects, and 2) no immediate solution to handle channel switching. I watched some Line 6 "dream rig" videos and decided to try the HD500X.
All indications were I would have channel switching and front end effects, then inset out to the DT50 preamp on to GForce...maybe even back to HD500X for some final EQ before heading back to DT50 power section. I really had no need for other amp models, very happy with the (4) I could access on the DT50. I envisioned being able to tweak and store presets using each of those 4 models, have compression and screamer on front end, then route to DT50 for preamp - then out to G-Force Chorus, Delay and Reverb, and on to poweramp....seemed a given. I worked the HD500X for several days with and w/o the editor software. Turns out if you want to use the DT50 preamp only - you must select "no model" inside the HD500X, then nothing else in the HD500X is available to use with the DT50...that is literally nothing, and using the Line 6 "Link" cable degraded the audio and amp tone to the point it was unusable. I could get some fairly decent clean sounds using HD500X blackface models and Line 6 link, though nothing approached plugging into the front of the amp. None of the HD500X dirtly tones came close to II and IV tones with my guitar cable straight into the front of the amp. All-in-all HD500X was a total disappointment.
Years back I owned a Triaxis, Simul 2:90 and used an MPX-G2 for effects. Awesome effects with an insert loop that allowed you to put the Triaxis in the middle with effects before and after it.....so I just bought a used G2 and R1 foot controller to try with the DT50 - again appeared this would do exactly what I needed (including relay output from R1 to switch DT 50 amp channels........it worked flawlessly. It actually makes the DT50 sound 1000% better, just as it did the Triaxis and Simul 2:90. I would highly recommend this solution to any DT25 or DT50 owner who experienced similar, and is thinking about giving up on the amp. Line 6 designers - you should do the same and try to emulate this solution (1990s Lexicon 24bit technology) in your next revision of HD500X firmware. Then you literally would have a "Dream Rig". Two DT50 112s or 212s with the MPX-G2 all in stereo would be truly "Heavenly Rig". Using a Tom Anderson Drop Top with HB in the rear. So I now plan to sell my GForce and Vetta II and should come very close to paying for the DT50 212, MPX-G2 & MPX-R1! Last problem is DT50 212 doesn't fit in the Vetta II combo road case.......minor detail.