You can calculate the required voltage and power based on specs or measurements, or just take a look at my table: http://www.head-fi.org/t/668238/headphones-sensitivity-impedance-required-v-i-p-amplifier-gain
Even if you add a couple of dB of excess gain you would be fine if the amp output only 400 mW into 32 ohms or 200 mW into 62 ohms for the K701.
I calculated it this way: K701 need 1.778 V from the table. Added 6 dB (doubling the voltage) excess gain => about 3.5 V.
P = V * V / R = 3.5 * 3.5 / 32 = 0.383 W = roughly 400 mW into 32 ohms
Without the 6 dB excess gain you need only 1/4th of that power. The requirements of my table are calculated so that a full-scale tone would be played at 110 dB SPL if there is no excess gain and of course you have to turn up the volume of the amp to 100%.
Edited by xnor - 10/15/13 at 12:14pm