Your "passive serial loop" clearly has (lots of) gain, so may not be very passive.
If you pad the signal on the way in, the output will be reduced by the same amount. This is as well, as otherwise the output of the unit would overload. If your loop does not have a return gain, it may be a problem.
While I personally wish we had put a line-in switch on the H9, we did not, so it should be used at guitar level. You might want to contact your amp manufacturer and ask them why they put so much gain on their effects loops – I suspect ours is not the only pedal which would have trouble with it.
