You shouldn't have a problem picking up the Delaware site unless you have a marginal signal from the Delaware site or you have interference on one or more of the voice channels associated with the Delaware site. I've only had the occasion to monitor the Columbus simulcast and the Delaware site on two occasions, but you likely won't be hearing all the same traffic on Delaware that you would hear on the Columbus simulcast.
I don't remember ever having a problem monitoring the Columbus simulcast site, at least not when coming in from the west on 70 all the way into 6th st near 71. I'm sure listeners are exposed to simulcast distortion, but I've never had a problem.
Your PSR could also be suffering from some overload on 800 mhz that might cause reception problems for certain frequencies you are trying to monitor . I don't know how close you are to any 800 mhz transmitters, but it's always a possibility. It has a hot front end but does poorly in a high RF environment.
If you get a signal on the Columbus simulcast that is full strengh, you might try setting that system up with the ATT ON. If you still get significant signal strength from Columbus simulcast with the attenuator on but you can decode better, it was probably an overload issue. Otherwise, if you still experience the problem with the attenuator on, you probably are suffering the effects of simulcast distortion or may have something other raunchy interference on one or more of the voice channels.
Mike