Thoughts on this:
1-Hardware inserts MUST use matching IO. This means, if outputs 5&6 feed the SPX, it must come back in on inputs 5&6.
2-IO used for hardware inserts PROBABLY should be removed/deleted from the input and output tabs(not sure as I do not use hardware inserts).
3-If the SPX has SPDIF IO, that would probably be a better choice all around(but do get it working on analog first). SPDIF would side-step a round-trip set of conversion.
4-Depending on what you are using the SPX for, it might make more sense to simply send to it, and return it to a separate stereo AUX or AUDIO track(do this for reverb/delay type FX). That would not require any routing config beyond assigning a send to a pair of physical outputs. An AUDIO track set to Input Monitoring will act like an AUX track, but you can easily record the SPX return so it is rendered in the session(would seem a good idea, at least for archiving).
BTW, anything in your IO setup shown in
italics is not a valid path and should be deleted/removed or corrected
