So I inserted shortening plugs into the input jacks and, well no noise at all. Not entirely sure what that means.
Can you explain what this means more fully...?
Sorry if I wasn't being clear enough. Yes, my PC motherboard is the audio source. When I said "other sources", I meant a variety of programs and websites, not
physically different audio sources. Again, sorry, I was indeed using "source" for software. Some of those "other sources" included things like Youtube & Spotify, and also some games I have like XCOM2 and CIV5, since I wanted to see if the buzz was just for a single program or for multiple ones.
I wasn't trying to turn down output and compensate for it by cranking the crack. I never turn it up beyond 1/3 of the way, or even 1/4, that's loud enough for most listening purposes. I wanted to make sure if the buzz (which sounded distinct from the regular expected hum and hiss) was actually there in more then 1 program and thus not just one programs issue. I'd turn down programs output to check because a few times I didn't notice the buzz because it was being masked under very loud default audio settings (Stellaris has some pretty loud intro music, it kinda blasts you). It turned out to be present in some programs but not others, which is odd.
To the other questions:
I am not using a DAC right now.
My setup is currently motherboard's line-in jack ---> Crack input jacks ----> Sennheiser HD600s.
I also finally got the wires delivered to connect my Crack to my phone; Besides there being more or less zero hiss or hum even with the cracked turned up to max (which surprised me), there was also zero buzz, from any programs or music on my phone. So if all the wires stayed the same but the only thing that changed was my physical audio source, does that mean it's just my motherboard that's causing the buzz?