The Schiit Modi is (judging from their own specifications) in fact more of a downgrade from the DAC of the Xonar Essence STX, and the ODAC is not an improvement either. You only really need a DAC "upgrade" if you have interference or ground loop problems. However, if you stop using the built-in amplifier of the sound card, it is no longer very good value for the money, and you may sell it and get the Schiit instead (which is probably still not bad) simply to save money. Alternatively, you could replace the STX with another, cheaper sound card that has no headphone amplifier, but still a good DAC (e.g. used X-Fi Titanium HD, or Xonar D1 or DX), and then combine that with an external amplifier. Sound cards tend to have good DACs, but the headphone amplification is lacking more often than not, and is never really great. Instead of the Magni, you may also consider the O2; not that the Schiit is necessarily bad, but the O2 has been tested extensively and is known to perform well with almost any headphone. The Schiit has the advantage of cost and aesthetics/better layout of connectors, though.