Assign all the active channels in the mix -- including MFX returns -- to a pair of busses (say busses 1 & 2). Also, make sure you take all the channels out of the L/R by turning off their assign button. Set the bus faders to unity gain. (You're soon going to find out that the busses do not feed into the L/R bus as they do on conventional mixers. That takes some getting used to.) Anyway, after you have all the channels assigned to the busses and unassigned (or is it deassigned, or are either one of those a real word?) from the L/R set busses 1 & 2 faders to unity (control + touch fader). Go to any open DSP channel, either on the Mic/Line or Tape Return bank, doesn't matter. Click on the word "PRE" at the top of the channel strip. A drop-down menu will appear, from it select "Channel Patch" and select Bus 1. Go to another open channel and channel patch Bus 2. Set the two channels to unity gain, pan them opposite and assign them to the L/R bus. You can now use the dynamics of those channels to process your 2-mix.
Alternatively, you can output from the bus outputs (using a D-sub cable) into your outboard processing gear and back into two channels on the Mic/Line bank, assigning those to the L/R.

HDR / D8B Bios Settings

Bios Microsoft word doc

Delay times for various D8B functions

A/D: 32 Samples
D/A: 25 Samples
Channel Strip: 18 Samples
So the Total A/D/A through the board is 75 Samples. 

Also, here are the sample delay for the cards:
PDI8 2 samples
OPT8 1 sample
DIO8(Opt) 6 samples
DIO8(TDIF) 1 sample
The AIO8 cards are the same as the boards A/D and D/A. 


Plugin Latency (all numbers in Samples) :
MFX - 102 
Antares Autotune - 403 
Drawmer - 158 
Delay Factor - 118
Time Pack - 118
Filter Machine - 118
TC Level1 - 118
GML Mono EQ - 177
GML Stereo EQ - 177
Saturated Fat - 118
RTA31 EQ - 118
RTA31 Analyzer - 118