I wouldn't worry about cables, as long as they aren't wired wrongly or broken in some other way. A amazon basic cable is probably fine or something similar to that.
schiit magni 3 and schiit modi 2 uber, but keep in mind you will need RCA to XLR cables to connect the pre out from the magni 3 to a pair of HS7. Just keep in mind you can't connect a mic or a midi keyboard to these.
It isn't a must for you to go out and buy a DAC too, but you will see some improvement. The HD 6XX are very good headphones for their price, they are afterall the massdrop version of the HD 650, which already were good for their price. If you keep on using your onboard then you will need a 3.5mm to RCA cable and use the audio ports on the back of your PC. If you want to get a DAC, then I will highly recommend getting the Modi 2 Uber over the normal Modi 2, since it is less sensitive to a bad USB signal and overall cleaner, plus not a lot more than the normal one.
You should set windows volume to 100% and control the volume via the magni 3. Also there really aren't any headphone that is officially studio grade, all of this "Monitor headphone" is mostly marketing.
