Hi Graeme777,
Running stereo in a setup where both amps have different settings or drive structures is not recommended. I understand many users wish to layer the sound of two amps live to sound better but whenever you introduce tonal disparities at the output (the speaker), the image ceases to truly be optimal for stereo audio. Think of listening to a mix in a studio but one monitor is setup with a different frequency response; we just don’t do that.
As you may have discovered, some algorithms mix or combine the signal of both sides to do what they do. In your setup, this forcibly mixes the preamp sections of each amp before processing. That’s what you’re hearing.
The only idea I have involves using amp modeling, where you can place the H90, or any effects, post cabinets after a mixer that combines the signal of both amps. Then, that connects to FOH. Many people do this. It’s like putting effects after you’ve recorded your guitar. The key thing to understand is that the signal from both amps was already mixed together before processing. That’s the only way to avoid the problem you’re encountering.
