For optimal control of the driver the headphone/driver pair should have a proper damping factor (ratio between headphone Ohms and amp impedance). No worries for degrading sound quality if the factor > 10 according to Innerfidelity, although this might not be a "universal" rule.
Some tube amps have a high impedance (as compared to solid state amps) making them not suitable for low impedance headphones. The WA7 that I'm about to buy has a switch for < 70 Ohms/>70 Ohm loads, and seems to work really well with IEMs even... .
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Edited by BDM-Fi - 6/13/13 at 11:39pm