You say that you have DT100's... a good choice and often cited as a studio engineers headphone of choice.
But did you know there are two versions?
The Beyerdynamic DT 100 Grey - 400 Ohm and the Beyerdynamic DT 100 Grey - 16 Ohm. And the different values of electrical resistance (ohms) makes a big difference.
There is a long article on the SOS website which explains it all much better than I can... Understanding Impedance... but to pick out the most relavant bit:
"Increasingly, people tend to use high-quality 'professional'-impedance headphones with portable equipment, and this is rarely a problem, except that the maximum volume will be reduced compared to a lower-impedance design"
So I'm guessing you have the 400 Ohm version. I would have thought that the Saffire 6 USB would be able to drive them but since you are unhappy with the volume then the 16 Ohm version might have been better for your setup.
Music Matter - Music Technology Store