I also compared connecting earlier Topping A90 as preamp. Sound changed minimally but bypassing it resulted in "cleaner" for me. Maybe configuration with Pre90 is another sound for ear.
I answered once but reread qustion. Low gain is also suitable. High Gain is just my preference. I think impedance (reactive part of it) proportion is fine.
30 may of 2022 I asked directly Bruno Putzeys via email with this question
I got answer:
Before this also were question from me.
And...
I used it in pre mode too. But here everything is banal. A specific analysis and service are needed for a specific breakdown. This requires services, knowledge, tools.
Why adjustment to crossover point is significant change? It can be anywhere. But resulting frequency response matters. I mean what the difference in crossover point if frequency response is seamless? I do not see any crossover joint.