Oh gawd. Not all this "more power = better" crap again... A 20W per channel amp and the Vali will feed the HE-4 (or any other headphone) the exact power value to sustain a certain volume. You want to get to X volume? It requires Y watts. Always, no matter what amp. The difference between the Vali and a 20W amp is the amount of overhead power you have and the architecture of the amp. You may like a higher powered amp better but that's because the pairing is more pleasing to your ears, it has nothing to do with power. As long as you have an amp that's got some overhead (i.e. as long as it isn't operating with the volume knob at 100% all the time), it doesn't matter. I'd much rather get a Vali and run it at 3:00 than get a mini-X and run it at 8:30.
I don't think thats the right argument. if your headphones require substantial power, like the he4 and 6, then more power does equal better. now, I agree that the difference between 5wpc and 100wpc doesn't really matter with these cans. but they do indeed need more then a typical headphone amp delivers. This is why I think people should try a nice Tamp with the HE4/6. they are very inexpensive, and deliver a nice tube like sound. worth a shot for what they go for.