Some recommendations from a DT990 Premium owner......
First, be absolutely certain that you are ok with the strong treble. You won’t notice on most well recorded, audiophile material but any modern, compressed, recordings will make your ears bleed. These are super strong in the sibilant region. If you are sensitive to treble, move along.
If you are prepared to deal with the treble or EQ out the 9k spike, these are actually very good headphones otherwise. They are very open, detailed and quite fast. The soundstage tends toward the three blob, left, center, right variety versus a cohesive front soundstage but, for the price, it is hard to complain about the other attributes. As any entry point to audiophile performance, these are a reasonable choice.
The other consideration is pro versus premium options. The premium has a much lighter clamp force and is very comfortable for long sessions where the pro might be a little uncomfortable.
If you dip your toes in to the audiophile headphone world here, you will eventually buy the HD6XX. It might be one month, two months or six months but eventually you will buy them. The other option is to skip these and go straight to the 6XX.