They mean, "doesn't make bad amps sound good." There's nothing about the HD 650 which makes them particularly hard to drive. It's certainly not perfect, with its 300-Ohm impedance, but there's nothing unique or unusually demanding about the voltage, current, or power requirements.
They can work well with low-end hardware. That is, if the output impedance isn't too high. The Behringer UCA-222 DAC (US$29) didn't sound particularly good with them, or particularly bad, but I didn't buy the HD 650's for headphones which sound "not particularly bad." The FiiO E5, when it doesn't run out of juice, worked remarkably well.