no need to get snarky here - that is not going to help.
your thread likely just got buried and missed and your question was fairly specific so unless someone had experience with both headsets they may have just avoided it.
impedance, sensitivity and frequency response do not equal better sounding. two products with the same basic specs could have far different sound signatures. having high sensitivity and low impedance generally means they are going to be easier to power while having more of a frequency range means they are capable of higher highs and lower lows. you may want to actually look at the charts (if available) showing the response curve as there will be peaks and valleys. what i mean by a curve http/graphs.headphone.com/graphCompare.php?graphType=0&graphID%5B%5D=713&graphID%5B%5D=533&graphID%5B%5D=2941&graphID%5B%5D=2881
though it may be hard to find for headsets!
for example two headphones could have a 20hz to 30khz response range while one of them is extremely bassy with little treble while the other is very bright with little bass. heres an example.. the dt770 is 5hz-35khz and the dt990 is the same. look here =713&graphID=2141&scale=30]http/graphs.headphone.com/graphCompare.php?graphType=0&graphID=713&graphID=2141&scale=30
and see that they will sound different. the dt770 is much more bassy with much deeper v-shaped signature. the dt990 is also a bit sharper on treble. curves are better than straight up numbers when judging headphone response.
headphone design like closed vs open also affects signature and soundstage immensely. open headphones generally are less bassy, do not isolate you from noise and have larger soundstages while closed are isolating, more bassy and with smaller soundstages. that is a generality and is not always the case as exceptions exist.
i know the mmx300 is overpriced and i know that it is based on one of the older iterations of the dt770 premium which as far as i know are less bassy than the current generation model.
is it just as good? that depends on your preferences. give them a listen and compare yourself and note what you think. there are some people who are more than ecstatic with cheap products and say they sound better than high end products while some people claim the opposite. what matters most is if you like the particular sound signature or not.
as for if a soundcard is needed, it depends. onboard audio is generally at a level nowadays where its fairly decent and good enough for most people. there are certainly benefits in quality to be had by using a soundcard but how much it matters to you and if you can hear it really depends on many factors such as how detailed the headphones are, the quality of the audio you listen to, how sharp your ears are and your own preferences for audio quality.
instead of a soundcard in some cases external amps or dacs are better but again, that depends on your usage patterns and preferences.