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A little late, but the treble control on the Panasonic won't do a thing for FM radio. FM radio doesn't carry any frequencies over 16 kHz. The treble control on the Panasonic operates at 20 kHz. That said, the HK is a pretty polite receiver, so the harshness you had problems with was most likely due to the source. The Panasonic, as you say, is very analytical. Probably a bit like M&K speakers, from what I've heard (no offense Dave :D).
Oh, and you can certainly get harshness and sibilince as low as 5 kHz, so the treble control probably wouldn't have done you any good, anyway.