I have been experimenting with using the Fender Hot Rod Deluxe as a tube power amp for the helix. I am starting to get some pretty reasonable tones so I thought I would share my approach in case it is useful to anyone else. Everything below assumes going from the Helix into the Power Amp In jack on the Fender.
For clean fender tones you can use the the pre-amp models like US Deluxe and US Double without much additional tweaking.
For dirty tones it works better using a full amp model (no cab) into the Fender Power Amp In. I typically setup a patch where I can A/B using a footswitch between my monitors and the Fender. The idea is not to get exactly the same tone, but it does help me tweak the tone going into the Fender.
My starting point is to use 2 EQ blocks after the amp model before going into the Fender. The first EQ block reduces the mids (-9dB) and treble (-3dB), but this should be tweaked for each amp sim. The second EQ block is a hi/lo cut (80hz / 15kHz) to remove the fizziness that is present in a lot of the high gain models.
I have used this approach with Cali IV lead, Brit J45, Brit, Plexi, and Tweed Blues and generally I am pretty happy with the tones I am getting.
I would love to hear any other tips from other Fender HRD users.
I still think I will end up buying a Powercab 112 Plus....