Er.. not owned one, but with the TiHD apparently the RCA out is the better internal signal to use. Connect to the external amp. Set for headphones and you're good to go. You'd have to use an amp though - just sticking the headphones into those ports would sound crap. Get an ultra cheap FiiO E5 or E6 for starters.
Read up the last 15 pages or so to find in more detail why you shouldn't use the speaker outputs for headphones (for surround). Basically you're making your headphones pretend they're speakers in front of you and not all around. Not optimal. Scratching my head as to why your Rca outs /ZxR with the 598 plugged in sounds so good unamped.
Lots of amps have that ability to output to speakers. There's loads! But the ZxR could've done it in software for you. With your new TiHD you'll have to crawl under your desk to swap then change the settings in SW as you can't have both headphones and speakers plugged in at the same time apparently (somebody correct me if I'm wrong on this).
Edited by SaLX - 11/13/13 at 12:43pm