Are you serious? For phony Advents? Where do you live? It seems to me a lot of people are getting rid of speakers that are "too big." I recently got some very nice Snells for free. And a CSP member has some AR-9s listed in the FS section for $800 which sounds to me like a steal if you have room for them and happen to live in New England so you can pick them up.
But to your point: I doubt that I know any more about xo's than you do but it seems to me that, since your Polks are ported, you could simply splice into the tweeter wires, run long wires out through the port and attach L-Pads, then adjust them from your listening position until you find the setting that sounds right. Then solder in your fixed resistors.
Kent
PS: Looks like you posted while I was typing but I still think my suggestion would work if you want a reversible fix