Hello sascrob,
My recommendation is to use the H90 in Dual Routing Mode. Configured this way, the H90 acts as two independent stereo (or mono) effects processors where Path 1 (I/O 1) can be used for your Pre loop and Path 2 (I/O 3) can be used for your B loop. On a program basis, you can put two effects in pre or two effects in post or one on each path. Be mindful of operating levels as most amp loops function at line level, but consult your amp’s manual and configure the operating levels on the H90 I/O accordingly. I recommend watching this video for a deeper understanding: https://youtu.be/0G_WtfUatSc?t=195&si=RctOC_fayhgq6w1p
