You might say the 280s are a bit more hi-fi. I didn't find them thin, but they did have more highs, and brought voices more forward than the heresies. If you listen closely to how the voices sound, you'll hear the heresy box sing with the music, where it doesn't on the 280s.
I had used an SPL meter when I tested the three speakers, and all were within 1db of each other. Perhaps placement or the impedance dips/amp matching of the 280s are making them sound less efficient or thin.
That being said, we all hear differently so my perception of better highs could be heard as thin. I did end up getting RF-7IIs though, which have more of everything that the 280s offer.