Today I compared two amps with the corresponding models on the Helix Rack.
My 1978 Marshall JMP 50w MkII non master volume (bridged), and my BRBS Vintage Class 50watt (https://www.brbsamplification.com/vintage-class-mk2) an Italian boutique amplifier, that is a 50 watt JCM800 clone (with some personal mods).
I linked the cab output of the amplifiers to a palmer PDI 03, a load box/speaker emulator, bypassing the speaker emulation and put the real "amp block" in the first helix fx loop.
Then I created a preset with the corresponding models (for the Marshall a Brit Trem Jumped, for the BRBS a Brit 2204) and a FX loop block with the true amplifiers, after that i put a cab block (a mandarin cab with a sm57 and a Royer 121, and then i created two snapshots: one with the model active, and another with the true amp.
After some tweaking I really managed to match the true and the model amplifiers, and they are really really close, also If I have to dial pretty different values on the modeled amplifiers.
In general, the amp models sound really really bright compared to the true amplifiers, but it's pretty simple to match the two without an EQ block.
I really recommend this experience, is an eye opener! The helix is fantastitc! On saturday I will have my first gig with the Helix, and I'm prette excited!