first, you're comparing apples to oranges.
the pc363d is very bass light and fairly neutral through treble and mids while the dt990 is v-shaped with peaks in treble and bass with significant bass extension for open headphones. i think before you start comparing products you need to figure out what kind of sound you expect to hear.
bass light? pc363d, adg1, ad500x/700x, k701
neutral? game zero, hd518/558/598, to some degree the dt880
v-shaped bassy? dt990, dt770
quality on the modmic is quite good for the price. youtube some voice tests if you are unsure.
depending on what you went with, yes you might need an amp. some higher end onboard audio can support hifi headphones but its hard to say to what degree as they all vary in output power. you might be fine with the dt990 on your onboard though if you want to get the most out of them certainly an amplifier is suggested. other headphones in the list above may be fine with onboard alone. i'd actually suggest fiio amps instead since they will be warmer for the dt990. the magni is very transparent so the dt990 will remain a bit cold if you used them. depends on what kind of sound you like to hear- both are good products.
the difference between bass light headphones and bassy v-shaped headphones will be immense and easily noticeable. this is why you want to think about what kind of audio you want before you go picking products out of the blue. as far as "quality" is concerned, they are going to be similar but there is much more going on than this and a simple word "quality" can not explain it all.
if your motherboard has virtual surround using just an amp will retain this feature correct.
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what you need to answer...
you need to let us know what kind of sound you expect to hear..
bass light & advantageous in gaming but less exciting and perhaps a little bright/tinny
neutral. more tone accurate and less exciting but great for recording or for people who do not like extremes
bass heavy. not advantageous but good for bassheads who value bass over treble/mids
v-shaped. peaked treble and bass which retains treble energy and detail while also boosting bass response for that exciting feel. basically a "fun" sounding can
also if you need closed or open..
closed isolate you from the world but have smaller soundstage and can get hot
open leak in and out but tend to have larger soundstages and also breathe so are cooler
is an amplifier a big deal?
some cans can be powered fine with your onboard while others perhaps not. is buying an amplifier a make or break deal? also do you need the cans to be usable on the go (portables).
do you absolutely require a headset or would this be your preference?
products like the adg1, pc363d, mmx300, game zero (all bass light and a bit tinny.. meant for advantage in fps games not enjoyment.. consider similar to ad700x sound) exist. the game one (based on hd598) is more neutral. there are not a whole lot of good "headphone manufacturer" gaming headsets out there... hifi cans often are superior in sound quality (other than the products i listed which are similar to the headphones listed by the name).