i've heard of issues with the headphone jack in monitors where it has caused distortion while in other cases it has worked perfectly fine. if you are having issues they should be rather easy to spot (crackling, popping, static, etc).
unfortunately what was stated above isnt quite true. not all cables, connectors and equipment is the same. while i agree there is no need for anything outrageous there is certainly a difference between "junk that doesnt work" and something "good enough". not all monitors have the same dac inside. some might be good, some might be junk. some monitors have a grounding or electrical interference issue, some dont. you can tell rather easily if its junk or if its good enough though by listening to it.
if you do not feel comfortable using your monitor output you could always get a hdmi sound extractor. this takes your hdmi signal, pulls off a left and right rca (red+white) female input and then sends the signal along to your monitor or tv. basically its a cheap DAC (digital analog converter). the left and right (red+white) can then be connected to your amplifier. they are normally only $20-30.
if you wanted something better quality yet you could get a hifi DAC with hdmi input/output but i dont think that will be necessary at all.