Here's an antenna that has always interested me, but because of my luck with passive loop antennas I've never spent the money to try it out:
http://www.ramseyelectronics.com/cgi...tion&key=SM100
It's tough to beat a good tube radio on AM. That's why GE and Conrad Juston decided to make the P780 which is often called GE's first Superadio. Maybe consider an upgrade to an R-390A? I know a broadcast engineer on the left coast who considers them the ultimate AM BCB DX radio. He uses an inverted L antenna with his, but then he's got the real estate for a +200' antenna.
Interestingly, his back-up receiver is a Kenwood TS-440SAT. Triple conversion, selectable filters, and great sensitivity once the built in attenuation is bypassed. I was pleased to learn this as my primary receiver is a Kenwood TS-450SAT.
Passive loop antennas of that size really won't help much with a Superadio or radios close in performance, but they do amazing things with less sensitive radios. Although I've used them successfully to block strong stations on adjacent frequencies with a Superadio.
Larger passive loop antennas will help. If you like to build stuff look around for plans to build a 2'x2' passive loop, and I think you'll be pleased with the results. I can't guarantee it'll help with the station you're chasing, but it should make a big difference even with a Superadio.
Also, it's not a bad idea to get your Superadio aligned. Although it seems to be less of a problem with the Superadio II, it can't hurt. I touched up my Superadio III and it made a big difference. The selectivity modification I made to it also helped to make it more sensitive and more selective than my Superadio II or any of my Superadio boomboxes. I also managed to get the dial to track a lot better. It's still off on the higher end of the dial, but it's much better than stock.