One other thing to try of course is the treble control. I would try turning it down a couple of notches. This depends on how it's implemented though. They aren't all shaped the same.
Usually any impedance over 10k is considered high and acceptable by most preamps. The more sensitive preamps will start to roll off high frequencies as the amplifier impedance drops or the volume control is changed. Look at a couple of reviews at Stereophile. In the measurements you'll see this effect.
No modern solid state amplifier should care however, and many if not most can tolerate down to 600 Ohms.
Usually any impedance over 10k is considered high and acceptable by most preamps. The more sensitive preamps will start to roll off high frequencies as the amplifier impedance drops or the volume control is changed. Look at a couple of reviews at Stereophile. In the measurements you'll see this effect.
No modern solid state amplifier should care however, and many if not most can tolerate down to 600 Ohms.