The "Fender Passport" model name seems to pertain to a number of different amps. Assuming it is the portable one, we'd be talking about 7W. This is a PA rather than an instrument amp, so its failure mode for drawing more power than it can deliver will not necessarily be musically useful.
However, what you describe does not sound like a typical overdrive reaction: most particularly it sounds as if vocals are reasonably fine.
So my guess would be that you do a solid bout of dynamic compression on the end result and the initial gain for voice is higher than that for bass. So whenever the voice comes in, the bass is disproportionally turned down.
Where does that dynamic compression come from? My guess would be some inadvertant setting: it would be unusual if the PA dealt with getting overdriven by turning itself down on its own initiative, in particular by mostly turning down the bass channel.
What would the symptoms be? One indication of overall gain getting turned down is also the level of white noise getting turned down: does the general noise level go down with the bass?
Another would be that moving the mic closer and further from the singer will not change the volume of the singer as much as it will change the volume of the bass.
So check out those compression settings if your amp has them: if you need compression on your voice, you need to do it with an external stomp box rather than generally on the PA (at least if the PA's routing options do not permit doing compression on the voice alone).