Audio Mixing

[Rogean]

Maybe someone here can help me select the correct product(s) to accomplish this, as I'm not an Audio expert.

Typical PC Setup with a headset using a USB Connection to 3.5 (I don't use onboard audio because it sounds like shit).

What I want to do is have a TV with a Roku, that I can hear through my Headset along with my PC Audio. To accomplish this, I need something that can take the SPDIF Out of the Roku, and the 3.5 from the USB Portion of my Headset, merge them and output into the 3.5 to my Headset.

I'm not on a budget, I don't want a small shitty $10 converter box, I don't mind paying for something that will work with 0 lag and interference. I start reading about things like electromagnetic interference, ground loops, and etc and I'm like da fuq. Also from what I read, SPDIF from the Roku comes in at 100% volume and can't be changed on the Roku side, so I need something with volume control.

So I don't know if this would need a decent audio mixer? What's the best mixer, what cables and adapters would I need.. ground loop isolator.. etc.

Thanks Pals