Dependin on the mixer, and the preamps
in it, you may not have any problems at all with this approach, or
you may need some sort of preamp/DI box between the bass and the
mixer.
Checking my mixer manual, I see the Tape input is designed for -10dBV
but the Line input can do anything from -23dBV to +17dBV. It does say
this line input is suitable for both instruments and high output
mics, and it has a little picture of a guitar. So I guess that
explains why the bass sound was OK in this case.
As the old saying goes, 'read the full manual'. I think that's what
RTFM stands for, anyway...
Cheers
Daniel