I hope I can be of help here, though I am a newby to bass guitars , cabs etc.
In the pa area that I deal in,'Gain' means the input level volume of the channel on the mixer and 'Volume' means the level of output from the mixer to the amp/power amp etc. Some mixers have fixed input gain to each channel, which are not adjustable, which can lead to a lack of sufficient signal strength to the mixer if the source has a low output ( such as guitars/basses etc ).
In the pa world, anyway, you can use any amplifier into any speaker. So, 1,000 watt amp into 100 watt speaker will go. However, 100 watt amp into 1,000 watt speaker might ( if you do not know the rule of thumb ) lead to both'blowing'. The rule of thumb, assuming that both amp and cab are of good quality, is to increase the output of the amp into the speaker(s) until the sound starts to distort or under stress. Then back-off the amp by a couple of clicks/notches or ,say, 1 hour on the clock. That should bring the sound back to'sweet' and allow both to play for the whole gig without blowing either. If it still sounds distorted or stressed, then back-off a bit more until it sounds sweet again. The reason for this is that the soundwaves go into'clipping', or 'distortion'. A clipped or distorted signal to the cab means that ,say, the woofer ( driver) in a bass cab gets a signal to move the cone forward & another signal to move it back before the cone has had a chance to reach it's maximum excursion. The result is that the cone heats-up( very rapidly) and melts or distorts so much that it starts to litterally break-up, and eventually fails. In this scenario, the amp will fail too, as the cone draws more and more power out of the amp which eventually melts or one of the components fails, resulting in failure of the amp.
Ok, so you can shoot me down now ! Not trying to teach anyone to 'suck eggs', though.
Cheers