Since I went through pretty much this exact scenario as well, I'll throw in and share my experience with it. I was using an old Mesa/Boogie 50/50 power amp in conjunction with the preamp-only model in the Helix. That worked pretty well, but man, did I ever get sick of having to lug that heavy 50/50 around. I'd heard good things about the ISP Stealth, so finally decided to get one and try it.
When I first plugged the ISP in and fired it up, it sounded AWFUL, and I was sure that I'd made a mistake in buying it. That's when it dawned on me that it was a 100% clean power amp, providing little if any coloration to the tone. Switching to the full amp models in the Helix (instead of the preamp-only models that I'd been using with the Mesa) instantly solved the problem.
A while later, I finally got to do the A/B test with my Helix/ISP setup, using the Cali Rectifire model, against a real live Mesa Solo Rectifier, using the same cab for both, and trying to match the amp settings and output as closely as possible. It wasn't a 100% identical tonal match, but let me tell you, it was DAMN close. Close enough to satisfy me, certainly for live purposes. Add to that the fact that the ISP weighs all of 1.2 pounds, and I'm as happy as I could be with it.