I don't mind spending $74 to support a good product and company, particularly one trying to push desktop audio at the frontier, so I bought the Audirvana license…it is worth it just knowing that I can get much better sound straight out of the MBP headphone jack. And if I can make my Fostex sound even better, then it's certainly a great value!
So...I never imagined my query would spur such debate, but after running it again with my Fostex HP-A3 there is now absolutely no question in my ears that Audirvana blows away iTunes, both without and with a DAC.
As promised, the differences are very similar in some respects to iTunes vs Audirvana through my headphone jack…brighter, better clarity and separation, wider soundstage, better bass extension and texture, crisper high frequencies. I've tried it with my Audio-Technica ATH-ES10 and ATH-A900X cans, and I notice a much larger improvement for Audirvana vs iTunes on the A900X, it seems to make up for some of their cardboard-esque timbre and lack of bass extension, without losing any quality in the mids (their greatest strength). The ES10 sound better also, it is just not as dramatic a difference as with the A900X.
Also, many knobs to turn in Audirvana in the preferences, I look forward to exploring it further than in the past.
Thanks for the advice!