I did have a digital signal
processor with a
crossover that handled this problem , but it's toast now.
Keep in mind that a
crossover between subs and mains is typically both low passing the subs and high passing the mains as well as possibly applying some delay to one or the other. And how you are handling functions such as delay for the fills, limiting,
etc, that the
DSP probably handled? Have you considered repairing or replacing the
DSP?
You could use a matrix
send for a
subwoofer, effectively very similar to an aux
fed subwoofer arrangement, but by feeding it only the main mix it would result in many the negatives and few of the positives of such an arrangement. For example, you could not take advantage of not sending particular inputs to the subs, one of the primary reasons for using aux
fed subs.
Also consider how such an arrangement potentially affects the interaction between the subs and mains. Think of a
line coming from the upper left to the lower right, that
line represents the slope of your low pass
crossover for the sub. A
line from the lower left to upper right represents the high pass for the mains (or say a roughly straight
line if you have no high pass for the mains). Where those two
line cross depends not only upon the slope of the lines and where the lines start (the filter corner frequencies), but also the amplitude (height) of the lines. Turn the mains or sub signal up or down and the
point where the related lines cross shifts both up and down and left and right, equating to the actual
crossover point shifting up and down in relative
level and frequency. It makes operating the
system that much more complex.
I'm also curious as to the left/
house and right/OH
stage monitor arrangement since as you pan something the
level in either output would change. Perhaps even more of an issue, any changes to a
fader would affect both outputs. Have you thought about using a pre-fader
aux send for the monitors?