You've got a higher output from the first combo than you'd have from the bass, nothing to do with ohmage, it is just gain stacking.
Take the effects send of combo #1 (which your bass is plugged into) and plug that into the effects return of #2 that should do it. The tuner, DI and headphones are outputs (as is the effects send) and the effects return and front input are 'inputs' so you could try combinations of out/in from #1 - #2 but don't use the Ext Cab option.
Does that make sense?