Well, a conclusion on this topic would be, that tubes may be a good match for k501. But I bought an OTL tube amp, the WA2, and I do not find it to be a good match. These phones do not shine, as I expected them to. It could be that I'm just not into the sound signature of AKG. A lot of people here on headfi have said, that k501 needs current. An OTL amp delivers a lot of voltage, and that's another thing.
That's why I'm selling k501. With the k601 pads, they were actually pretty cool, and there's something about their sound, that I can't get from any other phone, I've tried. I wish I was able to amp them properly. For now, I put my energy in the WA2 and phones that match well with that!