Just want to say that I agree. Case in point my AVR (4520Ci). The volume knob is an encoder, but it displays in Db. Depending on the input used the reading can be between -25 and -10 Db for an output that is quite equal.
I have added gain to the lower inputs of up to 12 Db and the higher ones of -3 Db to even things out. Now they are pretty close... mostly just so I don't get any surprises when changing inputs.
If you can get the max volume that you want without clipping, it does not make much difference if the pointer is at 12 o'clock or maxed out. If you need more at that point, raise the input level or gain.
Most volume controls only give a relative indication of where you are (dependent on input levels, input gain, and amp load).
Really the only problem is if the amp clips or starts to compress (sound stage falls apart) before you get as loud as you want.
560 hard to drive... don't know, I don't have them, but hope to one day. I do know that the HE 400 takes more drive (higher settings on analog volume control) to get to the level of most of my other phones, but so do my HD 600/650s. Of course they are 300 Ohms.