You're unlucky in that US stops flying the A330 after the summer and returns to the 767, leaving less potential Envoy seats.
We've flown the MAD flight a few times and notice that Envoy goes out full every time both directions ( Oct 2004, Jan & Jul 2005 ). A good chunk of those Envoy pax were Spanish nationals. I guess it reflects the market that US is building on the route, which is good for US.
As mentioned, you could hope for the best on the day of flight. But if you do have flexibility and can change the day, an Envoy seat could still be worth the change fee.
Enjoy your trip.
Barry