Update on the series filter configuration:
Everything works fine. This is now my standard. I tried it with three instruments.
1. Electric cello. Dry signal plus LPF on CH A, HPF on CH B
2. 5-string fretless with onboard low pass filters. Dry signal only CH A, HPF CH B.
3. 6-string fretted. LPF only CH A, HPF CH B
SF-2 is quite versatile.