I want to get the 250 ohm version of Beyerdynamic DT990
I currently own Fiio E07K. Will it be enough for this headphones?
If not, what amp\ dac \ tube I can buy for less then 100$ then will go well with those headphones?
If you want to get an idea of whether or not an amp will be able to drive a headphone, two specs you could look at from the headphone are the sensitivity and the impedance. You know the impedance, which is 250 ohms, and the sensitivity according to what I saw on Beyer's website was 96dB/mW. This means that at 1mW, the headphone produces a sound level of 96dB at a particular frequency, which, if I'm reading it right on their site, is 500Hz. Your Fiio amp can deliver 36mW into a 300 ohm load, if I'm reading the specs right on that site.
Regardless, my Pro/4AA are 250 ohms and 95dB/mW, and I can drive them decently out of my laptop, and of course they're louder with an amp. I don't own the DT990 250 ohm, but I'm going to go out on a limb here and say that I think you'll get enough volume out of setup. Maybe someone with experience with both units will come help you out even more.
Unless you're looking for an amp in that price range that pairs well with those cans. In that case, I can't really help you out.