Over the weekend I finished both the tube amp and the O2 amp. The tube amp sound is ho-hum. I'm not thrilled. On the other hand, the O2 amp is great. It neither adds or subtracts from the sound. It is transparent. It just amplifies. When there is no input it is totally silent on the output. No hiss or hum. As far as my ears can tell it is a really good device and I'm glad I built it. I bought the kit from JDS Labs. Their service is great. I have no connection with them except as a one time customer. Eric