is it better to volume the main device or the sound system?

I was debating this with my current setup, my PC's onboard sound busted and am using the video card's hdmi output to my tv which in-turn hooks up to my sound system (a 50+1 CD stereo with inputs). What happens is the sound system is hooked up through the headphone jack to the tv via a headphone to audio red and white split to the stereo. Note: the tv, when watching tv is usually set to about 20. The PC, the TV and the Stereo can all control the audio output, currently the pc is at 100%, the tv is at 50% and the stereo well it goes in -db so it is at -22db which is ok but I noticed if I lower the tv to say 20 and the stereo to 100% I hear buzzing... is it better to have the PC at say 50% then the tv 20% and the sound up all the way or is it better the way I have it? I'm asking cause my onboard sound blew out I think cause I listen to it at 100% with the speakers lowered to a decent level... so what is best? Also note, my other tv is actually hooked up to a real sound system and the tv controls it's own sound and has nothing to do with the sound system's sound level, simply cause it's plugged into like this Cable>System>TV, the system I'm talking about as of right now that I have the issue with I will again state, PC>TV>Stereo (last 2 being switched from the proper way). I will be getting a new sound card on thursday so I want to take care of it the best I can...