How to measure more than two channels at once?

[Frank PALM]

I want to measure my Left + Right + Sub combined output. How do I set this up...I only see the add second output on the Measure set-up window, but want to add more than a second. Using Asio 4All via HDMI and showing all 8 channels available.
Thanks

 

[John Mulcahy]

If the fronts are not full range sending a signal to left and right would usually redirect the content below the crossover to the sub.