Oh, I definitely wouldn't want to get rid of my HD6XX, even if I was "upgrading" to a better pair. Even though I prefer speakers, one of my favorite aspects of headphone listening is how easy it is to swap transducers for different applications and moods. And as many have pointed out, the HD6XX is a great pair of headphones that scales well with higher end electronics. Even if I was buying better headphones, I would keep the HD6XX around for comparison alone, especially since so many people have bought or heard it (including the HD650 of course) over the years; It's a staple in audio reviews to use as a comparison, so owning it makes reading those reviews and talking about headphones with random people (like we're doing right now) easier.
I do feel though that people get a bit defensive about them (or any value oriented devices). They are excellent value, which to me means they are
generally better than most of the options at the same price, and also
generally better than many headphones that cost more. But I've emphasized generally because sound is so subjective, and I don't like flat out "these are the best" recommendations for really any transducers. Secondly, while more expensive definitely does NOT mean better... it shouldn't be a surprise that a competent designer can make a headphone that outperforms something like the HD6XX when working with a budget that's twice or more as high.
My original comment to
@Rensek was basically just that if he did end up deciding to spend money on one of the best headphone amps around, I think it makes a lot of sense to also consider getting an upgraded pair of headphones to match.
To whoever is lucky enough to be close to the Shiitr, the best thing to do is bring your own headphones there. That way, you can easily swap between different amps and DACs with your headphones to hear what upgraded electronics will do for them. You can also then swap between your headphones and any of the nice headphones they have there on a specific amp/DAC to hear the difference that will make.
If you're not close enough, the next best thing is trying to find another audio shop that has a similar selection of gear from different manufacturers, or making it out to Canjam if there is one nearby. This is another thing I love about head-fi... You can't exactly do that with the speakers you have at home!