Don't get me wrong -- the headphone amp shootout that Absolute Sound ran a decade ago at this point influenced my buying and trying of the AudioValve and the Wheatfield, neither of which I was unhappy with, but I don't always agree with their conclusions (they preferred the E.A.R. HP-4 over everything else, IIRC, which I was least happy with, of that trio of products). And really, the only way to know is to buy it and install it in your home system for long enough to have an opinion stabilize. We're not talking a long time here -- 2 weeks to a month should be sufficient.
I mean, you can ask for Right of First Refusal from the person you sell it to (meaning they have to sell it to you at some future date if you ask for it back, usually at the same price, but at some predetermined price, and with some predetermined deadline [and of course within some predetermined timeframe -- I.E. you can't ask for it back in 50 years] [and there are, of course, other issues -- like what do you do if they end up not in the condition they were in, that needs to be predetermined as well]). But if that person is a distance away, then it's practically honor system anyway.
If I were you, I'd convince the wife that it really isn't a good idea to sell one before buying the other. Just make sure and promise her a month max or something (plus however long it takes to sell, if you're friend who wants to buy the Apogees isn't willing to do that for you).
You know how to read reviews, right? When he says he can't live with the Stax exclusively, but he can with the Quads, that's code for, "maybe not such a good idea". Seriously, I would make sure that's a decision I wouldn't regret before letting your Apogees go.