I think first you have to decide if you definitely need it to run both low & high impedance headphones. If you decide you only need it for the DT880, you can select something specifically tailored for high impedance. If you go that way and are at all DIY inclined, you could take a look at the Bottlehead Crack.
If you want something that works well with both low & high impedance headphones, you may be want to look something with a low/high Z switch or low/high Z headphone jacks. Have you read about the Torpedo? You can either build it yourself from a kit or buy an assembled version direct from Beezar.
Both of those amps are under $500.
Thank jboehle, so tubes is the way. My DIY skills are limited to solder cables but could be a nice challenge for myself, in the Bottlehead Crack info they say it's ok for beginners, I'll think about it. During the summer I can't keep a tube amp close to me, I'm thinking to get something like an A1 Gold clone as secondary amp, should work fine with the Ms1i and 940.