Yeah they tend to retain their value because of all the hype surrounding them, the he-560 are in no way a worse set of cans though.

You can find a used pair of he-560's for around $500, and those are some amazing headphones.  I heard the Oppo PM-3's are pretty good too, but those go for around $600 new, might be able to find them on sale somewhere though.

The headphone amp on the scarlett 2i2 is actually pretty good, same with the dac. I don't think you will be lacking anything in terms of soundquality. As an added bonus, the pre's are also really good so your guitar is gonna sound great. Source: owner of a 2i2