You could try the FiiO E10. It is cheap, includes a DAC, is available in your country at www.headphones.cz, and should work fine with the DT990 Pro if you do not need very high volume. Alternatively, if you only want an amplifier, the E11 is slightly more powerful and portable, but can only be powered by batteries (and therefore needs to be recharged regularly). For a lot of power driving high impedance headphones at reasonably low cost, you can use the (AC powered and not portable) E9/E09K.
Edited by stv014 - 1/20/13 at 9:47am