Please excuse me for the following geek-speak. If someone posts with direct experience, don't bother reading further. If not, here goes:
Output impedance specs are problematical. Some manufacturers quote the "source" impedance, others spec the minimum "load" impedance. A good rule-of-thumb is that the latter is typically one-tenth the former in modern, "voltage-matched" systems. So a SS preamp rated at 600 Ohms output (source) will usually be quite happy with a power amp input at or above 6K Ohms, which the Stratos is (10K). And, should the Adcom spec be a "load" one, that means 600 or greater, so there's even less of a "matching" issue.
But Adcom's (web site) specs don't distinguish between the "active" and "passive" modes. And I would certainly expect the "source" impedance to be well above 600 Ohms in the "passive" mode, as it must be to maintain the stated Adcom input impedances without causing substantial losses. What does that mean for interface with the Stratos? Perhaps little - read on.
Should the Adcom "passive" output impedance be well above 600 Ohms, that's not likely to present any serious issues if you typically run the volume control below 2 o'clock or so. The loading that the Stratos input places upon the Adcom will, at such rotation and below, mostly cause some variation in volume change versus control rotation characteristic, with respect to the "active" behavior of the same preamp. So what! If it sounds better that way, who cares? If you must run the control above 2 o'clock, then more of the Stratos load may reflect back into your source equipment. This shouldn't be an issue for most SS source gear, but it could be for some tube stuff.
One old "trick" in volume control design is to use a low-impedance loading resistor across the output of a mid-to-high impedance volume control in order to achieve the "audio taper" (i.e., more-or-less uniform volume change with rotation). If the Adcom employs this, it would work fairly well as long as the volume control stays below, say, 2 o'clock (again). Rotate farther and the source sees a too-low load impedance and starts having its own problems, especially extreme bass rolloff. This is a kludgy way of doing things and I'd expect more sophistication from Adcom.
Probably the best thing to do is to try the Adcom in both "passive" and "active" modes with the Stratos. There's not likely to be any problem whatsoever in the "active" mode. Passive? Who knows? Listen and discover. You're not likely to have problems with solid-state sources in either mode.
