That's odd. The NFB15 drives my HE-500 with ease at high gain. At 12 on volume pot I get ear damaging levels. Even at low gain it gets very, very loud at 3 on volume pot. The harder to drive HE-4 gets pretty loud at 1 on high gain. The NFB 11 and 15 shares the same amp section so....
Check your computer settings, drivers, etc.
That surprises me, too. I upgraded from an O2 to Asgard 2, and it was more powerful (to me) than the O2. Then I upgraded to the NFB-11, and it is bit of a step up from the Asgard 2.
That being said, a 10x increase in watts is only +10 db more SPL, and a +10 db increase in SPL is a perceived doubling in volume. So depending on what your expectations are, might not seem a lot more powerful. You might want to read this article, kishorfarm: http://geoffthegreygeek.com/amplifier-power/