Hi Max!
This depends on what you’re trying to achieve and what your current configuration is, but for a simple stereo send/return you do not need to use Dual Mode. You can use any available outputs on your 18i8 to send audio into inputs 1&2 on the H90, and then run outputs 1&2 from the H90 back into any available input on the 18i8. You will need to do some configuration in the Focusrite Control software to avoid creating a feedback loop.
Which outputs on my audio interface should I connect to which inputs on the H90?
I would suggest sending outputs 3&4 on the 18i8 into inputs 1&2 on the H90.
Which outputs of the H90 should go back into which inputs on my audio interface?
Since you’re using inputs 1&2 you can then send outputs 1&2 from the H90 into line inputs 5&6 on the rear of the 18i8.
Which input/output configuration should I select on the H90?
You should use the default Insert Mode, and switch your I/O to Line Level. Just press and hold the Programs and Routing buttons down for a second to access the System Menu, and then scroll to I/O, press Select, and change In Levels and Out Levels from Instrument to Line.
And how should I set up the routing in Ableton Live?
First, you need to configure your Focusrite Control software. Assuming you’re using Focusrite Control 2 you will want to change the source for the outputs you chose (3&4) from Mix or Analogue inputs to Playback (DAW) and choose Playback 3-4. Then, in Live go to Preferences, click Audio, then click Input Config and make sure inputs 5&6 are enabled and named, then click OK and to go back and click Output Config and make sure outputs 3&4 are enabled and named, then click OK. Once that’s set up just go to Audio Effects in the Library column and grab an External Audio Effect and drag it onto an unused Return Track (Return C, for instance) in Live, and make sure you select 3&4 in the ‘Audio To’ field, and 5&6 in ‘Audio From’ field. Now, anything you send to Return C will be sent through the H90 and back into Live. You can set the Return track up to be Pre or Post, whichever you prefer. Next, in the Options drop down menu in Live, make sure you have the Delay Compensation option enabled. Lastly, since you wish to use a Return in Live, go to the System menu in the H90 and under Global turn to Page 2 and set Kill Dry to On. This will ensure that the H90 only returns the wet signal back to Live so you can blend this in with your tracks.
I’d also suggest keeping the volume very low while getting this all set up, just in case you accidentally trigger a feedback loop.
