Hey there, welcome to HIfiguides, So I may be able to offer some help or information in some sort but I would have a few questions before I can do so. Got a bit of a wall of text here sorry about that but quite a bit to cover and ask on this one lol. Hope that’s ok.
I would first ask, is this your first brighter headphone purchase? The DT 990 is extremely sharp on treble and very bassy with vocals pushed in the back due it’s recessed mids. The SSS sounds people make can be extremely uncomfortable for a lot of people. If this is your first bright headphone and your set on the 990 I would highly encourage making absolutely sure there is a good return and refund policy just in case it is too much. I would hate to have someone pick up a headphone, have it be painful to the ears, and not be able to return it. Keep in mind, since this is for competitive gaming, that the dt 990 is very bassy which can get in the way of fps sounds as footsteps and the like. It’s still good but there are better options in this area and it’s definitely not the best Daily driver for casual use so to speak.
If its literally $100 more? No, it’s not particularly worth it. While you do stand to gain better sound from 600 ohm and more refinement in the highs, it’s not particularly required and you will need a strong amp to power the 600 ohm much more than the 250 ohm.
By combo do you mean a Combo unit like a 2 in 1 unit? For this kind of headphone you would want something that can roll off that treble peak. Something clean and neutral is fine if you aren’t treble sensitive otherwise you definitely need a particular amp as 990 is pretty particular in this regard.
So, my question would be is this for Consoles? If so, that limits the choices you have at your disposal by quite a lot as you need an optical port for console use. You can get away with seperate units as there’s quite a few dacs that have this port but for combo units G6 and a small handful of others are the main ones for choice. I don’t particularly think the G6 is good unless this is console gaming… its decent but not really all that great honestly. If it’s pc gaming I would definitely have a completely seperate recommendation.
Generally, dedicated units(meaning seperate amp and seperate dac) will provide better results, sound, and power. Granted they generally will cost a bit more than a combo unit.
So, schiit is a bit of a strange one. They have good equipment but they are hard to get outside of the USA. There are viable alternatives if you are looking at seperate units. If we are bringing the Magni 3+ into play thats a $100 amp… the alternatives to the magni are JDS Atom, Topping A30, and Monoprice Liquid Spark granted I may be forgetting a couple but those are the ones that come to mind immediately with the Spark over there being the best for brighter headphones such as the 990 followed respectively by the magni with atom and a30 being the more neutral clean sounding amps.
Mics have multiple options. Modmic is one of those. DT 990 and its brothers 880(semi open back) 770 (closed back) can actually be modified to have a detachable cable allowing use of 3.5mm attachable boom mics such as v -moda boom pro provided you can do the mod yourself or find someone who can do it for you. Other options are like you mentioned a desktop mic or a lavalier mic which is a clip on microphone for your shirt, cable, etc.
Lol, it’s quite alright. There’s a lot to understand in audio. Take your time find what works best.
As far as further recommendations… I would have to ask for answers to the previous questions listed before I can actually further recommend anything… I do have a guide on here over in the competitive forums if you want to take a look. Falenkor's Competitive Headphone Write Up. Now with added Hardware explanations! it may help explain a few things, provide some extra guidance, and also have explaination to the ideal sound for a competitive gamer headphone and why it is that the style of sound is more ideal. You of course have “all rounder” headphones which may be what your looking for instead in this case due to regular video watching but I feel it’s worth mentioning that… Note that guide was written a bit in a hrry and should be taken with just a general sense of minor recommendations.