I've been playing my U4 non-stop except for sleeping these last 3 months.
I leave the tuner on the local jazz station for the cats during the day while I am at work.
Anyways, I tried a new rectifier, the mullard ez35, and the back wall of my room disappeared. It sounded as if the whole room grew by ten feet in each direction(on two axis). I thought WOW this mullard is the bomb, then the A/B scientist took over. I put the stock tubes my U4 came with in and let it warm up. Same thing. My room sounds wider and deeper, the 3d fill has had a nonlinear growth. I am amazed at how the low level listening soundstage is now, I used to have to turn it up to get the best imaging. No more. I love low volume listening. I can't wait for the Norcal circle to hear this little gem again.