I have the hd500x and the DT25 also. I've been playing around with the hx effects with both. First of all, you can put your hx effects in the chain with your dt25 in the 4cm and it will work great. You can even connect a midi cable from the hxfx to the dt25 to control everything as your specified. If you are heart set on using the tubes and cabinet settings on the DT25, I don't think I would even connect up the hd500x. But the crazy thing is you can..... connect your hxfx with your hd500x using the 4cm and use your line6 link to your dt25.
I have been creating patches for 4cm on the hxfx and the hd500x and it sounds fantastic. I put the extra send/return from the hxfx back to the hd500x return so that I can utilize the amazing stereo effects, and I can't believe how it sounds with headphones. I haven routed the hd500x to multiple amps yet for stereo effects, but I plan to when I get some time. I'm envisioning doing some wet/dry/wet stuff with these tools. Here is my routing plan for the hxfx and hd500x for 5/6 cable method.
Guitar -> input on hxfx
put mono effects like compressors and eq and drivers here on the hx effects.
put effects send/receive block in the hxfx
route the l/mono send output to the guitar in of the hd500x
place any other pre-amplifier effects you might want on the hd500x think volume pedal limiter etc. (How about a dual amp setup now that you don't have the DSP limits!)
put a the effects loop block after the mixer on the hd500x
route send mono or TRS splitter from the hd500x send back to the return left/right on the hxfx
Now put your awesome delays and reverbs in the chain after the send/recieve block on the hxfx
route your left/right output from the hxfx to the return of the hd500x if you want (not necessary if you are going to take outputs from hxfx directly to frfr amps)
If you do, you can use headphones on your hd500x, route outputs to dt25 with or without l6link or other amps using the outputs. The possibilities are unlimited here and lots of crazy things you can try.
Now, imagine that you put a impulse response block after the return in your hxfx. I haven't tried that yet, but that promises really amazing possibilities.
I haven't tried controlling the hd500x from the hxfx or the reverse using midi yet. But that is on the horizon also.
Bob