You purchased possibly the least efficient headphones that you could! Excellent rep' that Beyers have for sound quality they have a lowish sensitivity of 94dBSPL (for I assume 1mW in?) most phones are at around 100dB, This 6dB loss would hardly matter but that it is compounded by a much higher impedance than most modern "cans" of 400Ohms. 32Ohms is a common impedance these days with 100Ohms being about the upper limit. You will find very few interfaces that will drive those phones to high levels and precious few budget mixers. You will also need to be careful what you buy in terms of a headphone amp (for that is the only way if you want to keep the DT100s) but I am sure good suggestions are being thought of as I type!
Regarding the mic and recording levels I suspect you are trying to record too hot. Digital is not tape. You should be running 24bits and 44.1kHz and the software meters averaging about -18dBFS,peaks no higher than about -8dBFS.
Getting computer, AI and recording software happily married can be a bit of a trial but at least you have come to the right place!