Well done

I see youve owned the HD 518; how do they compare when it comes to pure SQ, detail and value?

It is hard to tell, since I have never owned both at the same time.
My guess is that the 518 has better soundstage/imaging, and better timbre/tonality - my last item in the "con" list for the 449 does not apply as much to the 518. The 518 are probably less even in response from bass to treble - they have the classic Senn "veil" and are very easy to listen to for hours at a time, partially due to a treble rolloff that prevents the typical screechy treble response of many modern recordings from becoming tiring.

Thank you very much for the response! Headroom states that the 518 has much and much more treble (above 10KHz), is that also the truth outside the Headroom studio? I mean, is the difference really big?
I love the extra details you can hear in songs, for instance those ''faults'' in the records which are in the treble frequencies. Also I listen to soft music only (classical, Leonard Cohen, Il Divo etc.), but I like the extra bass my HD 205 II gives me (like the extra punch of a drumkit or the bass of a string in an orchestra). Will use it only with an amp though, maybe with the Sansa Clip + sometimes.
Does this all mean the 518 is better suited for me than the 449?