I am also interested in this - I would imagine that it would work as you described, with the stereo outputs from the Helix HX Stomp connected to the stereo ins on the firehawk 1500 amp and a mono cable from the guitar out to the guitar input on the amp, but you would need to setup your patches on the Helix so that they send a dry signal to the guitar output and stereo with effects etc to the stereo outputs, is this possible on the Helix HX stomp?
I have a Firehawk 1500 and the FBV3 foot controller, so im wondering if the MIDI out from the amp was connected to the Helix's MIDI in, would this allow patches to be selected and effects to be switched on/off using the FBV3, also would it be possible to use the expression pedal on this controller for Helix effects?
If this works in this way you could just have the Helix Stomp pedal on top of the Firehawk 1500 connected with short patch cables and control everything from the FBV3 foot controller - could be quite a cool setup ...