I am not sure what brand tubes Dan is using in his preamps but the original tubes in the 9.0 were Philips 5687 tubes. Those tubes were late 1970's or early 80's tubes and frankly, not the most musical. If you are the original owner be happy that the tube lasted a year. The 5687 tube runs hot and like everything else man makes, it breaks. Like the others before me, yes, check the tubes first. Having owned a 9.0 I ditched the stock tubes and went with Raytheon 5687 tubes and never had an issue. I did have an issue with the preamp itself though and mine developed a hum or a whine after it was on for about 90 minutes or so. It had to go back for repairs. It did not do it when I first owned it. It started after about six months and fortunately is was still under the warranty. Since this preamp was in place for awhile and there were no issues previously, I do not think you have a ground loop. I think your amp is a Class D amp and I have had impedance issues with my tube preamps and class D stuff. But then again, you said you have been using this system arrangement for awhile so again, perhaps not an issue.
Go with tubes. And if you buy NOS tubes for the preamp, buy from a reputable dealer. You might pay a little more but you won't be spending money on junk either. I have seen a lot of counterfeit tubes from certain parts of the world and the tubes that people think they are getting are nothing but rebranded POS tubes.
