After lots of learning from others here, thought I would give back with a fun work-around for getting a USB based FX Loop (or send / return) working with the Stomp.
I have been playing with the HX Stomp for the past week trying to get a setup that allows me to send a signal to an external Laptop for processing and then return it into the block chain. I want to make this as resilient to failure as possible, so having this as insert inside the Stomp allows me to bypass in the rare event of a system crash. I am using Ableton Live for some wacky Max4Live processing, allowing me to use the Stomp for more generic duties like modelling, dynamics and modulation effects.
I am running my stomp straight from my Bass guitar, and into a clean amp. All blocks need to be mono for this config to work.
Source (Bass Guitar) into Mono (L) inout of the Stomp, and then Mono (L) output to amp. Its important to pan your main L/R output path in the final block to Left, as this free's up the Right output for the return from the USB feed.
Create an FX Loop R Block. Keeping trails on and then assigning a CC controller (exp pedal) to the Mix value will really make this fun and flexible.
Add a small patch lead from the Send Output (I just used a standard mono jack guitar lead) to the Right Input of the stomp.
On your Laptop and chosen Application ( I am using Ableton here), select the Stomp as the USB interface and set the processing track/channel to use USB Input #4.
Add your desired DSP plugins etc and then route this channel out USB Output #2.
Connect a Patch cable from the Stomp Output Right to the Return Right.
All I/O in my instance was set to Instrument level, and the send and return levels matched the internal Stomp block path really well, but you can adjust this in your DAW or the Stomp FX Loop block settings.
The scope for fun this way is vast. I was trying out some pitch tracking tools and MaxForLive patches, converting the input into low synth lines, then goofing around with some granular clouds. By being an FX Loop block on the stomp, I could move this around to either feed or be fed by the Looper Block. You can even split the FX Loop into a Send and Return to increase your scope for mangling.
I measured the latency with a Max Patch I have made, and the Stomp by itself measures between 1.4ms and 1.7ms depending on the blocks chosen. Adding in Ableton Live, with the buffer set to 256, this added another 15.9ms. You can use lower buffers, but you will get the occasion buffer underun glitch. As I am mostly generating 100% wet FX within Ableton, and the Stomp is processing the direct signal, you get the best of both worlds.
I have used Ableton on hundreds of live performances, and have to say that its amazingly stable - it has crashed only once in that time, and that was due to a naughty third-party convolution plugin, that swiftly got replaced by the Max convolver, so I am not too concerned about stability. Having the Stomp as the core to my sound and now as the Audio Interface into and out out Ableton is just amazing!
If the Line 6 developers / product managers are listening in these forums, it would be so much cleaner to extend the USB Audio routing to allow this configuration without resorting to turning the Stomp into a cable-octopus. Ideally this audio routing would then allow us to do this FX Loop via USB in Stereo.
Attached is a pic of the Stomp and a blank Preset with the FX Loop block and routing done. Have fun.
FX Loop USB.hlx