Hi Axel
the optimal placement of a digital fx processor, providing time domain fx, is between preamp and power amp. If your amp has an fx loop you should use it. If it doesn't…. well…it doesn't.
Levels should be taken care of in programming the presets. Your ear should judge what you perceive and adjust levels accordingly.
Once you have set your I/O levels, under the LEVELS key (this are system parameters and can't be stored for every preset!), you should use the preset(s) levels, found under the second PARAMETER keypress window, LEVELS.
FX A and B LVL are the outputs of each single FX block. Routings can severely change how levels and ear perception work. If a preset uses a serial routing, the FX A level will influence the FX B level since A > B. A parallel routing will work differently as each block is completely indipendent.
You may well need to do some tweaking and ear testing with your presets anc check for the best levels. Their values can be very different from preset to preset, so don't be alarmed if you read very different dB values as your ear is the boss here.
all the best
I
