Just like to post an update on this - I shotgunned all the resistors today since I ordered up a batch anyway. The nasty hum in the AM band is gone (despite the resistors checking ok - probably had one that drifted as it got hot), and I get some stations, though they're really noisy. Probably because I'm using a really short wire, but if it doesn't improve it may need an alignment. Either way, problem resolved!
