668b is relatively inefficient phone and requires high power.it is so sad that I need to return this ALO Audio Amp, it really make HD668b sound different, thicken the low freq and make high freq absent, make HD668b dark...
And it is very linear so that at high volume you can enjoy the phone to the fullest extent with high power amplifier.
Aune X7s is one of them, and is not expensive.