Best to call Rogue for a chat - they’re friendly! Personally I would definitely be concerned by this bit "3.3k ohms at 20Hz" and this bit "the balanced line-output impedances were twice these values". That means 6.6K at 20Hz from the XLR outs?! That’s way WAY too high for a preamp IMO. Even if on paper it doesn’t look like a big dB loss in bass, and 20Hz seems far enough down, in my experience is you’ll be able to hear this as a leanness in sound.
And yeah, at least with prior models (Athena, Hera) the input impedance of the preamps changes with gain setting, so other stuff might change too.
Quite frankly if I had this unit I’d be tempted to try the headphone out (via adapters) as the preamp. The headphone out has VERY low output impedance. It probably makes some compromises to get that (e.g. lower quality coupling caps to achieve the low ESR), but I’d argue it’s probably less of a compromise than a 6.6K output impedance!
Also keep in mind Rogue's power amps often have a very high input impedance, 100K - 300K. That might work OK with the 6.6K, but when you use other amps...