There was a lot more than 20dBu on the rooftop for both stations.
The one antenna was looking right at the Pomona translator on 94.3. It was a large Scala yagi with about 11dBi of gain. On a clear day, you could see the tower in Pomona from Fort Lee, especially through binoculars. It's amazing how far very little power on FM can go with a perfect line of sight. There was about 40dBu of signal coming in there, add the gain of the yagi and the fact that it was in mono and it was a nice quiet signal.
Even the 99.7 had about 38dBu of signal. The "hot spot" on the rooftop was behind 3 grounded 10' roof vents, that were 10' apart, and lined up perfectly with Empire. They acted like a big reflector and were serendipitously positioned almost 1 wavelength apart. 94dBu - 40dBu for the cavity - 35dBu rejection on the radio put 99.5 at 19dBu on 99.7. With a received 99.7 signal of 38dBu, it actually did work. Some days were better than others on the direct feed. Like I said, 90% of the time it was repeating 94.3, but for a back up it worked fine. The other part of the secret was running it in mono. The radio didn't need a lot of signal for 50dB of quieting.
There was a small amount of "cha cha" that bled through on the audio from 99.5, but with program material other than talk it wasn't noticeable. Luckily, we had quietly modulated neighbors, not Z-100 or Hot 97.
The current setup is receiving 94.3 exclusively.
106.3 is just a bad frequency for that location. 95.9 would have been a better choice, farther away from WFOX, WVOS and WRAT than anything on 106.3 is. Unfortunately, that's not a minor change to jump that many channels.