as you can tell mixed driver setups are greatly discouraged.
but the one thing that can make mixed drivers work, and does make mixed drivers work is a crossover. lucky you the carvin amp has a active one...yay
But yes your correct you should use biamp mode with your cabinets.
The low output would go to your 1x15 and the high output would go to your 4x10
so yes the output of a 1x15 is much lower than a 4x10.
this is why carvin is smart and put a level control for each output. so you can balance the system.
start with the level turned up on just the 1x15, then bring up the level on the 4x10 to match.
once the system is balanced use the master volume to control the overall level.
Keep in mind at lower volumes the levels on the high/ lo might be different, than using higher master volume settings. So when you increase the master volume you might have to re balance the hi lo levels on the crossover.
the 4x10 can handle a good deal of bass so you should be able to use a lower crossover point, even down to 200hz
this means everything above 200hz will go to the 4x10 and everything below 200hz will go to the 1x15.
of course this is adjustable so you can sweep the crossover point till it sounds good to your ears.
start slightly below the midpoint around 700hz and then lower it to 600, 400 ,300 till it sounds right to you.
again as said the weak point is the 1x15, you can balance the system but when you need to get louder and the 15 starts to distort, then that is max volume of the balanced system.
most likely as stated the 4x10 should be able to go louder, but you wont be able to go any higher than this if you want to without adding another 1x15. Or just running a matched system without the crossover and just run 2 fullrange 4x10's or 2 fullrange 1x15's
also you could try just running the 4x10 without the 1x15 and see if that gets you more volume than using the balanced pair.
if you find the balanced system not loud enough in your band situation.
also if you just listen to just the 4x10 on the crossover and sweep the crossover point higher say 600hz to 700 to 800 to 1000hz
you will notice that it will be reducing the amount of bass content going to the 4x10 since it is a highpass filter (only high frequency pass through)
reducing the bass will also reduce the overall level since most bass guitar is below 700hz. by using a higher crossover point it can help reduce the level of the 4x10 if you need more adjustment between the High low adjustments.
so say the lo crossover is way up to get the 1x15 right and then the high is turned way down to get the 4x10 to balance right. Raise the crossover point to help further reduce the level of the 4x10.
most likely since the 4x10 can handle alot of bass the crossover point will be around 200hz. but then again there is a limit to the system since now most of the bass content is being reproduced by the 15, and again as mentioned the balanced system can only go so high because the 15 will distort first.