I know I'm late to this party, but I've been using the Power Amp In on my Katana 100 with my Helix LT and so far I like it. To be fair this is tiding me over until the Human Malware crisis is over and the band can start playing again. I hope to have a Powercab+ by then but still plan on using the Katana at home as possibly the other speaker for stereo fun.
To OP - yes that is how I do it as well. Instrument cable from the LT mono out to the power amp in on the KTN, and use amp with no cab in my modeler.
I've found that using preamp only (at least in Helix) can have unpredictable results depending on the model. Volumes are all over the place and reverb and delays respond differently. I just use the full amp but no cab and I've been able to eek out some really good tone to my ears. I'm hopeful that compared to the PC+ with speaker modeling the KTN will pale, but right now it sounds pretty darn good!