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The thing is, the various mfr’s of gear all want their proprietary stuff to become the ‘new standard’, so you end up with quirky equipment, especially in the lowend market, plus, you’re rarely dead sure of which end of the connections to which ratings apply. They have thus contributed to the confusion of referencing I/O impedance, which is of more concern than bal/unbal connections, unless you’re talking about long runs or around wiring and/or equipent that can be emitting EMI/RFI.
I have the big Yamaha book, “Sound Reinforcement Handbook”, which, BTW, is a ‘standard’, and hasn’t anything especially relevant to Yamaha-only gear … it’s very generic. Even though it says ‘Reinforcement’, it also includes a great deal about audio recording (relative of broadcasting); includes just about everything there is to know about how various pieces of audio gear hook together, matching input/output impedances, balanced/unbalanced lines, lots of diagrams … it’s a great book. IMO, no studio should be without it on hand. If I really need to know about something involving audio signals, it’s my first choice. It’s getting worn.
For example, in it, (pp 87-88 for those who may have a copy available) you’ll find a section on ambiguities of source vs. load impedances. What we can say is that load impedance is almost always at least several times that of source impedance. So if you see a high Z rating, something over, say, 1400 ohms, it’s more likely to represent load rather than source, regardless of how it reads in a manual where only one end is rated.
In the end, you might need to find out some things on your own, take measurements, write to mfr’s, etc. BTW, concerning the phase-reversed insert, I haven’t received an answer from Yamaha so far.
That said, my opinion is that the phase-reversed Insert will not have a detrimental effect on the hook-up we’re considering, but rather only on outbound-only signals going to other gear, e.g., using only the output portion of the insert to go to a separate P.A. system, or something like that, which I doubt you would want to do anyway.