i'm not sure why you pm'd me requesting a response here as the correct response has already been given for both of your connectivity options, but i'll clarify a few things and list my thoughts.
i did not find a manual on the website online, so please consult the manual in the box to double check how they require connections. i did find this video https
/support.steelseries.com/hc/en-us/articles/224942187-How-do-I-setup-my-Siberia-840-
it looks like for pc they suggest using usb, 3.5mm and power. i'm not sure if the usb is for mic or for an onboard soundcard. if its for mic, the below statement is probably. if its for the soundcard it may or may not be true. simple fact is if its analog 3.5mm input its not greater than 2.0 though if its usb i cant at a glance say without testing or knowing the specs.
i read a review on amazon about the 840 which suggests that using usb you only get dolby pro logic surround and that to get true surround you need to use optical. an example of pro logic is upscaling a 2.0 signal to 5.1 in a similar way to how virtual surround works but at the same time different and not quite the same. i'm not sure how true this statement is or if the poster has no idea what they are talking about since i dont have a manual, full specs or the headset to test.
while optical is suggested for consoles it is also possible to use it with pc. you may want to check about 5.1 support via optical on the 840 end but external soundcards with an optical jack with 5.1 support meant for home theater hookup would output it on the pc end. i believe the xonar u7 has ddl based on some comments i've read (do double check) and i'm certain there are many cheaper cards which do as well. if your headset supports 5.1 or greater via optical and you have a soundcard with said support things should work perfectly fine i would think.