My guess is that is likely why they haven't been restocked yet. I've been watching for a restock for close to a week now. Hopefully they'll be back up in some form soon.   I just caved and bought an Asura 2.0 in the meantime. :D

They've replaced the HD600 for me (as well as quite a few others) just simply because of the total package they provide.   The sound quality is quite good and I agree with the comparison to the HD600 (they probably fall somewhere between the 598 and 600 sound quality wise), they're inexpensive...

Those are some nice cans. I rock a 598 myself for my gaming/music needs.   I think the not using onboard comes from the fact that you can never tell what kind of sound you're going to get from it. Some are ok but a lot are just not up to snuff for driving a lot of headphones. The 598 shouldn't...

If you want surround your best bet is the Astro. Hard to beat if that is what you're after since basically everything else on the market other than gaming focused amps are going to be stereo. 

Can you demo any of them or are you going to have to buy them without hearing them? My advice is always go with what sounds good to you first.   If you can't demo the amps the Fiio is a solid amp and so is the objective. I'd start with whatever fits your budget more.   The Astro may be...

Re-introducing myself since it looks like all my posts are gone and I've been inactive for a long time.   Got quite a lot of cans and looking to be involved in the hobby a bit more actively again!   Hope to get to some pacific northwest meet ups!