I have compared it to a Woo WA22 and a Singlepower PPX. As a headphone amp, it is the closest you can get to a tube-like sound but with terrific bass and phenomenal left/right imaging. I think it sounds almost as good as the Woo, which was a lot more expensive.
I listened to the Rudistor and the Headamp at a meet. The Rudistor did not impress at all. The Headamp sounded good but a little dry for my tastes. I don't think you would be unhappy with Headamp. I have not heard the Violectric, etc. If you are thinking of spending $1000 on a headphone amp, I would go with tubes, no questions asked.
I think the Blue Circle sonically is a great amp for the money. It makes music. Some solid state amps I have heard don't. But the Blue Circle is a strange looking amp. The money was put into the circuit design, not a glossy chassis. But I remember the days when visibly beautiful Singlepower amps were all the rage and have learned from my experience with Singlepower that beauty is sometimes only skin deep.