I would turn the LP Filter up to 100 so as to not have it interfere with the avr crossover, other than that it is up to you and the SPL meter to decide if the volume level of the sub is set as loudly as you like it, 5 is a good place to start if you think it isn't loud enough just turn it up a little more. I would also take a guess that the other input bypasses the LPfilter on the sub leaving you only the volume adjustment working.

Jason
M80 v2
VP160 v3
QS8 v2
PB13 Ultra
Denon 3808
Samsung 85" Q70