There is no hidden global output parameters. The only global levels parameters are input/output levels, which are adjustable from instrument/line level. If you are just going bass guitar > H90 > amp, it would probably be best to have these all set to instrument level.
Also, be aware that each Preset and the Program itself has input/output gain levels. Please make sure these are all set to 0 dB and the H90 should not be changing the level of your bass by a noticeable amount.
