I have all three. I use the A5 on the go, the BL when I am on my laptop, or at home, and the SM2U sits forlornly in the corner, wanted to join in...I mainly use the Schiit for comparison purposes, now. As for "driving power," the A5 falls behind the SM2U, which has less than the BL. The later two, on their highest power settings...
How the three utilize that power is very different. I think the A5 is quite efficient, but does tend to suffer at higher volumes, kind of like trying to keep up with the big kids. It is quite nice, though. The BL drives REALLY well, even at high volumes, and is slightly warm to me. The SM2U? Excellent at any volume, but does run a bit on the hot-side to me in regards to treble. Of course, that can be EQ'd out. Both the A5 and the BL give better adjustments to me, and get the majority of use in my abode.
FiiO A5 is an excellent pairing for on the go. And on sale fairly often.