Just to add what Alan is saying about high output impedance and how there can be frequency response variations. This is in terms of portable music devices and headphones and not tube-amps and speakers but the concept is similar.

Ipod Touch 3G - 7 ohms (High)
Sansa Clip+ - 1 ohm (Low)


(NwAvGuy, 2011)

http://nwavguy.blogspot.com/2011/02/sansa-clip-measured.html

I’m armed and I’m drinking. You don’t want to listen to advice from me, amigo.

-Max Payne