the amps loop has a dry/wet balance but 100% is still parallel! It’s not possible to make it serial.
It’s possible that effects loop functions differently than other amps with balance control. ENGL amps, for example, have a balance control and 100% generates a serial FX loop according to the manual.
There is a 50Hz noise when the H90 (in dual mode) is connected like this: Guitar->H90 input1 H 90 output1->Duotone H&K input DuotoneH&K send->H90 input3 H90 output3->DuotoneH&K return In the amps clean mode it’s not a problem.
This specific frequency is indicative of a ground loop in the system. You can try placing an isolation transformer in between the amp’s FX send and the H90 and the H90 Out to the effects loop return. It’s possible only one of these will solve the issue.