…i do, too, run H90 through the FX Loop of one HX Stomp – and while in my current setup i have not experienced an issue similar to what you describe with it, i have it clear as sunlight that the same artifact is happening in my kit with my (pair of) TC Electronics Ditto X2 looper(s) are plugged, series, into the FX Loop of a second HX Stomp – when its Mix control isn’t set to 100.
i have figured this issue as the result of a phase problem (for the techies among us, read my lips here, please: i deliberately wrote phase, not polarity) that stems from each device’s own latency, in my case.
even if my 2 loopers do not have a (proper) dry/wet control (only a playback gain pot), the artifact you’re describing matches my experience in full.
i could trigger the same result (as a proof that my rumination had purchase) by summing two different signal paths after a really modicum amount of delay (~0.2ms, if i’m not tricked by my exhausted memory) was added to only one of them… exact figure might differ from my recollection, and i apologize for this in advance, but it is that small amount of time addition that creates a partial cancellation of multiple specific frequencies – comb filter describes it quite effectively, within the perimeter of my limited vocabulary.
it explains, to me, how wisely Eventide have chosen to add a delay control onto the H90 Insert Routing path: had my own two HX Stomp had such feature within their FX Loop, i would have had proof of my theory in a few second’s time (instead of weeks of mumbling).
bottom line is, i have a table full of digital stompboxes, which i daisy-chain in really long sequences for my sound design practice, each of which requires (beside the time that its internal processing takes) also the time needed for Analogue-to-Digital conversion, and that for Digital-to-Analogue, too… latency, simply said, comes from the sum of these processes.
i’m aware of veering to a most extreme approach in my signal chain vision, but that’s why i’ve been longing for AES/EBU digital-in/out-only FX pedals for years (though i know all too well that they won’t ever exist, in stompbox format).
