Your receiver does not have treble adjustment? That's essentially what Tyll did with his phone.
Beyond that the Sampson is $60. That's the same price as the KRK KNS 6400 here:
Get the 6400 while the price holds.
If it does have treble adjustmant, I don't know about it. It's an older dolby digital receiver. But I wouldn't want to have to mess with it every time I use headphones anyway. Then set it back again after. So do you think I should pass on the HD681 and HD668 then? And the KRK you mention says it's a closed design.