Due to it being an otl amp, it's optimized for high impedance headphones. Don't bother with Grados, for instance. I don't view this as a con, but merely as a design characteristic. Truly synergistic with my HD600's.
I've used film and teflon bypass caps to bypass all critical caps, and padded down the volume control as in stock form, it's way too loud by the 10 o'clock setting, also using beautiful NOS tubes. So no,,this isn't a stock Crack, but if you've got the DIY leaning, this is a great project.