The Xonar STX is worth every dollar ~$180 in the long run, so in the end I've decided to place my order on it instead of 1. my existing DGX soundcard + external amp or 2. External DAC+AMP All-in-one unit or 3. Upgrade my existing sound card with better Audio chip and DAC built in + external amp. The Xonar STX option is far better than the above 3 scenarios I mentioned in terms of performance to price ratio and upgradability.
Xonar STX is known to have a high end DAC that's comparable to some ~$1,000 DAC external units as most people say, and the built in headphone amp is pretty decent and capable to drive higher impedance headphones. Take $180 and divide the price by 2 and that's like $90 each for DAC and Amp for desktop/stationary use. The value is much better than any of the 3 scenarios I came up with above. Plus, you could always upgrade the 3 upgradable op-amps on the card anytime down the road to keep you happy and satisfied from the itch again.
I'd say that a O2+ODAC is a good alternative, but when comparing 180 dollar to 280 dollar, I'm not sure about that the value for money would be better in that combo. However the amplifier in the STX is somewhat unsuitable for easy-to-drive headphones, so it probably comes down to a question of what headphones you're going to be using. Prices in different countries can affect which alternative that would be the best suitable for your needs and use. With all that said, you're probably going to be happy. But the fact that you can change OP-amps pretty much seems more to be side-grades, rather than upgrades from everything that has been written about them in this thread.