Personally, I think it's the driver.
Reason I think this is, it works for the 32-bit version of MC but doesn't work for the 64-bit MC and that suggests to me the driver doesn't actually (fully?) support 64-bit... or in this case, 64-bit Media Center. Believe it or not, 64-bit Media Players on Windows are pretty rare, with the most popular still being 32-bit (e.g. foobar2000, Windows Media Player, etc.). I believe there was a discussion not too long ago about ASIO drivers lacking 64-bit support, if I recall correctly).
You mentioned there's no more development on the drivers, how long ago was the last driver update? Honestly with no new driver development, I doubt it'll work with the ECHO.
EDIT: Looked into it a little bit, the last driver for the Audiofire series was released back in 2013 (so it means, for example, it lacks official Windows 10 support). I wouldn't be surprised if it doesn't end up ever working with the 64-bit version of Media Player. I have a device with drivers that support 64-bit media players with ASIO, and it works fine. I don't think it's a bug in MC, but an issue with the driver.