As long as the output impedance is <1/8th of the headphone's, you probably won't notice too much of a difference. I'd leave that to your ears to decide. Usually, lower gain = cleaner and less distortion and noise. When comparing the two, make sure you volume match, because louder = better to our brains .
http://www.head-fi.org/t/707281/headphone-data-sheet-table-with-power-voltage-current Looking here at the DT880, with similar power requirements, you can see that all versions need about 200mW to reach goal peak levels for volume. So you'd be fine with any of the impedance models :).
To do the math for calculating power requirements (extra credit for the A+ student here): You need to know the headphone impedance, and sensitivity rating. First, look at the sensitivity. It's usually in db/mW. Take that number, and for each additional 3 db in volume increase you want, you double the power [(10*log_10(2 for twice the power))]. So if a headphone is at 90 db/mW, to get 93 db, you need 2*(1 mW) = 2 mW. To get to 96, it's 2*(2 mW)=4 mW. To get all the way up to 120 db peaks, you end up at 1024 mW, which is basically 1 W of power.
So for the DT990's, which I think are at 96 db/mW, you need 256mW for 120 db peaks. Most people only need 110 peaks, which lowers the power requirement to 32 mW. Easy. Then, you can use rated specs of your amp and the power equations to figure out expected power into different loads (assuming the amp is an ideal voltage source... lol). So if an amp does 2W into 32 ohms, you can use V^2 / R to convert between the two. 2 = V^2 / 32, which means the voltage is 8V out. Then, just put in your new headphone R(impedance) value (250 ohms let's say), and you'll get Power = 8^2/250 = 0.256 W = 256 mW. Plenty of power :).
The reason the power specs on the Vahalla 2 don't match up with that math is because it's not an ideal voltage source from an EE standpoint. For that to be true, you want an ideally 0 output impedance, which OTL tube amps don't have (Output Transformer-Less). Also, many amps become limited in how much current they can produce, which affects lower impedance headphones that suck up a lot of current. That's why the Vahalla can put more power into the bigger loads.
As you can see, the DT990's are actually pretty easy to drive. Many high impedance headphones have a high sensitivity, which people don't account for. Inversely, some headphones with a low impedance have a low sensitivity. Many planar magnetic headphones are like this, as well as some dynamic driver headphones like the AKG K701.
This concludes your EE lesson of the day :).