Hey - thanks for the tip on the HD600 deal! I really appreciate it. $239 brand new is a very good price for these 'phones.
I have had them for a week now, and after some A/B on some very familiar CDs, I chose to return the HD598s, and keep the HD600s. There was a lot to like about the 598s, but in the end, the 600s IMO have far more accurate and musical bass, as well as less colored mids, and a more realistic sound stage. The 598s are a fun and comfortable set of cans at a value price, but I feel the 600s better meets my goal of a reference quality, neutral signature - cans that I plan to keep for years. My source is the Onkyo C7030 CD player - very nice for < $200. I did headphone comparisons using the Onkyo CD player and my 41 year old Marantz 1060 integrated amp, using it's headphone output stage.
This leads to another set of questions - I have budgeted $250 for a Schiit Asgard 2 headphone amp. But, I am finding my old Marantz through it's headphone jack sounding quite nice in driving the 300 ohm HD600s. The 1060 is all original, with no new parts. All still works as it should (for now) and sounds very nice. I am not at all familiar with the design of dedicated headphone amps, and how they differ from more traditional amp designs.
Would there be appreciable benefits in sound quality using a dedicated amp such as the Asgard 2 over my vintage Marantz integrated amp? I do notice just a touch of roughness on the top end on some material so far with the 1060. Does the Asgard 2 sound smoother or in any other way different than my 1060? I have not really considered a tube amp or a hybrid at this point.
I love the compact size of the Schiit, and the apparent high quality for the price. I could return within 15 days per their policy, but thought I would ask input of others first.
Thanks for the great advice, all.