Was that what they were called, RadioDX10 ? The Soul of Philly?
I caught them once in a snowy supermarket parking lot outside Elysburg PA, about 2 PM, around five years ago. That reception was off a Grundig too, like yours, maybe. This was a SR-270 or something like that -- the black one with the big red on-off button.
The music was fine. Philly's Spinners, Hall & Oates, Intruders, etc. I guess that WDAS 1480 sent some daytome signal NNW toward us.
I heard 'em once at night, too. They send a bit of nighttime signal our way.
They must've been quite the thing in the 60's.
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Model 450 DLX 'field radio'. That was it.