I think it's personal preference more than anything. I use only flats on my HF1s, could go either way with the HP1s and absolutely hate them on the HF2s.
I find the HP1s still have enough bass with bowls and appreciate the slightly more expansive stage and better detail retrieval that I perceive with the driver slightly further from the ear.
Dusty, one reason I asked was digging up an old Hirsch post where he stated very categorically that the polarity switches caused the HP1s to roll off far more quickly than the HP2s at both ends of the frequency spectrum. Now, I don't hear anything particularly out of the ordinary with my pair, but then I don't rate either my accumulated headphone experience or my ears particularly highly.