ASUS Essence STX ii with Sennheiser HD598 Headphones Setup - Am I doing something wrong?

I just switched from a Sound Blaster Z card to an ASUS Essence STX ii due to Windows 10 constantly setting my audio to 5.1 on a whim, which was severely messing with my audio in most circumstances.  The audio quality overall sounds pretty solid, but after reading a bunch of suggested setups on forums, I've found that a rather basic setup in the ASUS Audio Center seems to have the best sound quality for me.

What I'm currently using:
2 Channels
PCM 44.1 KHz
Headphone
NO Dolby Headhpone or 7.1 Virtual

If I up the channels to anything else, it gets this sound like someone's playing music in a tunnel.  The best alternate setup I found was with the Dolby Headphone & 7.1 Virtual turned on, but it was still kinda echo-y.

I do a lot of gaming, movie watching, and music listening on this PC.  Of top priority is the gaming, as if I really want to just jam out to music, I've got a Sonos setup in my living room.

So, am I doing something wrong?  Should it sound echo-y?  I can honestly say I don't have very good hearing (military service, lots of gunshots, motorcycles, industrial & agricultural machinery, and meniere's disease doesn't help with that), but I can still (usually)  tell good quality from meh-quality audio, and the fact that I can't seem to make this card sound better than the SBZ is throwing me off a bit.

Did I mention I'm running Sennheiser HD 598s? 

Thanks for the help.