Rather than mixing down the the front/rear channels for the sub, I'd like to try splitting the bass for each of the ranks thus routing the bass notes to the two subs at a natural break point for both the front and rear speakers.
I don't use the "rank split" facility myself.
However from reading the HW 5 manual...
1. You need to create a Mixer Bus Group which includes the "speakers" you wish to use for Main/upper part of the ranks, lets call this "main_group".
2. You also need to create a Mixer Bus Group for the bass part of the rank, lets call it "bass_group" which includes the "speakers" you wish to use for the bass part of the rank.
3. For those ranks you wish to "split" on the Rank Routing to Audio Mixer(primary) Bus Group for multi-channel output panel
a.You need to assign a Mixer Bus Group to the Main/upper part of the rank(s); assign the Mixer Bus Group "main_group" to Rank output 1. Select the bus allocation algorithm you wish to use (I would use "Static:cyclic with octave, octaves cycled). Make sure that the other Rank outputs 2,3 & 4) have their destination mixer bus group set to a mixer bus group which has no primaries in it (<none> in my examples).
b. Click on the "Bass part of the rank" tab at the top of the screen (and open the rank allocation for the Bass part of the rank). This screen is like the other allocation screen but includes a section at the top of each Rank Output section where you specify the "split note number" (see HW 5 manual pg 191).
c. For Rank Output 1 assign the Mixer Bus Group you have created, "bass_group" as the Destination mixer bus group; select the note where you want the split to start (Notes above the split go go the Main/upper Mixer Bus group set up in a, "main_group" and this note and the notes below go the ot Mixer Bus Group set up on this tab, "bass_group".). Select an allocation algorithm for this rank output. Make sure tha the other Rank outputs 2,3 & 4) have their destination mixer bus group set to a mixer bus group which has no primaries in it (<none> in my examples).
You need to allocate each rank you want to "split" in this way. Remember you can use multiple select (cntl+shift) to select several ranks which you want to be routed in the same way.
I would not use a sub woofer for the bass part of the rank alone - you need a full range speaker to play harmonics (sounds) which the typical sub woofer will not play.
Perhaps you could use three speakers for the front main/upper sounds and one pair plus the sub for the bass/lower sounds. For the rear I would send a mix-down from the rear primary (two rear speakers) to the sub.
If I get a moment I'll send you a suggested configuration....
The level of sound going to any speaker / sub may be adjusted on the Primary Bus or master Mix Bus panel...
Iain