@jb61264 so you have a mac or windows computer
if you are using a mac you can set up an aggregate device in macOS and then use that as the i/o for your DAW
if you are using a PC there is something called ASIO (I think) that may do similar,, but I'm sure I've heard it's not as good as the Mac
NB botht he ES8 and ES9 can be used as audio interfaces as well as for cv/gate/modulation
with both the ES8 and ES9 you can go direct into and out of them at modular levels - so there is no need for an output module
if you live in a country with balanced power you may want an output module to go to the 8i6 and an input module to raise the volume to modular levels (something like veils will do the job perfectly well - check maximum gain of exact module before buying)
although an attenuator may also do the job perfectly well if needed as an output module
my general advice is to start with nothing, if you get clipping try an attenuator in front - they are inexpensive and useful - if you still find it too noisy try an output module - but unless your outputs are balanced and your inputs are balanced then I'm not convinced it will make any difference
"some of the best base-level info to remember can be found in Jim's sigfile" @Lugia
Utility modules are the dull polish that makes the shiny modules actually shine!!!
sound sources < sound modifiers < modulation sources < utilities