... Unfortunately the line level output is too low when there is very low room gain....
Piece of cake (and almost certainly appropriate) to control the input to the amps, not the output to speakers (which has no piece of cake at all).
BTW, the real issue with the parts in a passive crossover, should anybody be interested in such outdated technology choice anymore, is to buy extra duplicates of the capacitors and coils so as to choose the best matching ones for L and R.
B.