if you plan to keep an xlr microphone, you would want to keep the 2i2. speaker amps and headphone amps generally do not have 48v phantom power for microphones.
if you plan to keep the dt990 you would likely want to keep a headphone amp as speaker amps are rated much higher. there are better amps and dac on the market (schiit magni/modi stack is better than the e10k) but that is not what you were asking. its possible to use some types of headphones on the front jack of a stereo receiver as well but its often not suggested as much as using a dedicated headphone amp
active or passive? depends on your preferences honestly. actives take up less space and require no external amplifier. there are great choices like the cr3,cr4, m-audio av32, av42, micca pb42x, rokkit 5, yamaha hs8, etc on the market depending on the amount you wanted to spend. many are great although they may lack a little on the low end given most are just smaller bookshelves. there are interesting 2.1 kits like the swan m20, m50 which might work or you can always add an active sub later on. passive speakers require a seperate amplifier but you have more choices. the one problem is that amps tend to leap from budget level units with reasonable pricetags to expensive units with high pricetags with very little in between. units like the dta-120 are great for a budget and will power many speakers to reasonable volume levels but if you were looking for something stronger an av receiver is likely the better choice. as far as good speakers are concerned, i personally like my klipsch and the horn design really is vibrant. i've heard great things about diamond 9 and 10 series over in the uk as well.