Another common misconception is that headphone's impedance directly correlates to its sensitivity. K1000 and K501 both have 120 ohm impedance but the sensitivity differes by 20 dB. Sensitivity is usually specified as dB/mW. Having a lower impedance allows the amp to pump more power into the cans at a fixed voltage (P=V^2/R). Usually, lower impedance headphones are also designed to be more sensitive, but that's by the engineer's choice, not because impedance determines sensitivity.
Back to the Grado and Cayin HA-1A. Several people have posted that they don't hear any noise. Maybe their unit is quieter than mine or I just have more sensitive ears, I don't know. On my own amp I used some low-noise tubes and the hiss is reduced to an easily acceptable level. I don't use Grado to listen to music with a lot of quiet passages, such as classical, so it does not bother me much.