Len, I'll have to drive west far enough to get away from WBBF (oops, "WROC") on 950 to see wazzup IBOCwise on 940/930/920.
Bob 1370, at the risk of getting pedantic: "Is this situation fixable without really nasty, costly retro-fitting?" The answer to that is "it all depends." The issue is "pattern bandwidth" and related "common point linearity." There doesn't literally have to be anything "wrong" with an AM system in order to have problems with IBOC. Stations with old DAs that have common-point impedance curves that look like pretzels can still sound fine in analog - you might notice, in high signal fields, a kind of "Donald Duck" or single-sideband effect momentarily as you drive through nulls and the carrier drops in relation to the sidebands. This is a minor annoyance in traditional AM listening but can be disastrous with IBOC. If the sidebands and carrier aren't in precise relationship to each other the self-interference problem becomes acute. This is why many stations can't utilize IBOC such as the former WCFL in Chicago on 1000 kHz. "Retrofitting" might require actually tearing the whole array down and starting from scratch, a multimillion-dollar proposition that just isn't worth it.
Would such measures be necessary with Thumpin' 1370? Who knows? Let's see now....as I recall, that array was built by....Gordon P. Brown!! All I can say is: Be Big, Be A Builder!