There is a difference between "powerful enough to drive" and "drive it to it's full potential.
Planar are usually very power hungry.
But the btr5 can drive my Era-1 ( 20ohm, 94db/mW) to nice listening levels out of the 3.5 se. Didn't try the 2.5 bal with the Quad.
With my Monitor 5 ( 40ohm/ 99db ), i can't barely go beyond 40 out of 60 on balanced without the chance of loss hearing.
My Audio GD R28 on the other side, gives the power to play with autority to their full potential.
That doesn't mean the R28 is able to reach higher volume levels ( it does ^^), it's more the dynamics and control, which brings more satisfaction to myself.
Your EL-8 will fit perfect balanced.