It's definitely the empeg.
When i change the EQ settings, even just switch the presets to and fro, the sound returns to normal. My bass amp is of better quality and has a 1.5 sec delay when powering up. It is quite definitely there. the front output seems to be attenuated until I change the EQ settings.
I have a EQ preset in 4 ch mode, front OP to highs, rear OP to sub. The fronts are affected.
I don't know if it is just a high freq thing as the sub goes thru a LPF before the amp.

ni!

Murray
Go-Wit-Da-Fro