I had a Pod xt Pro (with all the model packs installed) and tried it as an outboard effects unit with my tube combo (both 4CM and in the loop) but couldn't get it to sound good. Using the drives via 4CM was just awful. Loop effects were better but not great. I have a Nova System in my back up rig and I really rate it but the time based effects are pristine sounding and don't have an analogue vibe. The OD and distortion on the Nova are analogue but, as you can't use it with 4CM (there's no loop on the Nova), it does reduce the useability a bit.
I recently bought a GSP1101 and it's the best box I've tried for integrating with other pieces of gear. I've (so far) used it in a rack with a couple of different preamps in the loop and it worked flawlessly. I'm currently using it via 4CM with a 35w tube combo and, again, it's integrated perfectly. Some of the delay settings on the GSP have a nice analogue vibe (for a digital unit) and the compression and drives work really well via 4CM. If you go for one, it's definitely worth updating the firmware to c63 from the mustbebeta site.
I like to use (stereo) detuning. How well would a dedicated device in the loop for detuning work with the 1101? For example, if I got a Lexicon MXnnn for detune and used the 1101's chorus/mods, would the loop be assignable in the right position in the chain?
and finally, to integrate real stompboxes, would I want to use something like a GCX?
Speaking of GCX (self hijack here), what are some good alternatives? Looks like the GCX runs on CC numbers rather than program changes/presets....not exactly what i have in mind.
Not certain about your detuning question but the 1101 manual is available on the Digitech site and I'm sure some of the more knowledgeable members here will know the answer.
The only alternatives to the GCX that I'm aware of are the RJM effects gizmo and the Rocktron patchmate, which comes in either rack or floor versions. As far as i know, they both work on midi program changes.