A few weeks down the line now, I feel I have to make an update on my previous statements. Yes, the iDo is still a great amp and it has no trouble driving my HD600's. But I cannot say that the tube amp doesn't make a big difference anymore. My Little Dot MKIII is now equipped with Mullard EF95's and Electro Harmonix 6H30Pi Gold Pins and I have done lots of hours burn-in (except for the Electro Harmonix's, they only have 6 hours or so). And the difference is substantial. Deeper, much deeper and more controlled bass. With the iDo alone I thought the HD600 was very gentle on the low end, now I know better. It has great bass with the tube amp. Plus, the sound stage is so much wider with the tube amp. It is as if everything comes out of the headphones and is really outside, around my head. It is still not in front of me though, as I would have wished, but it is still really beautiful.
So, don't get me wrong. The iDo is great. But upgrading with a tube amp is really a step up. And then upgrading tubes is also a step up. All small steps but comparing the end results will not go unnoticed to anyone.