No offense but I don't think you measured correctly. 45-50 dB would barely be audible over the steady state background ambient noise level of most homes. To get a quieter background you would need to do some soundproofing.
For you head-fi guys, if you ever see some tubes I'm selling on eBay and you want them, PM me here. I will NOT ship internationally without the Global Shipping program on eBay to just any random person because I have been burned too many times and it's just not worth the hassle. But for you guys, I will ship directly (if you pay by paypal of course) without any eBay involvement.
One other suggestion...with the amp off and cool of course, try re-seating the tube. The problem you describe can sometimes be due to a poor contact in the tube socket. Make sure that you see both filaments glowing. Then as others have suggested, with the volume all the way down and without valuable headphones connected, let it run for a good while.
If after all that it doesn't seem to be working, do contact the seller, who I happen to know has a 100% customer...