There may be something wrong with your preamp outside of tubes-There is no way it should sound as poorly as described if all is well. What kind of amp are you pairing with? Have you played around with the current jumpers? If it is a tube issue, there are plenty of options out there ranging from the inexpensive to more pricier options. I have played around with a number tubes, including 6sn7 tubes w converter. I am currently using Siemens CCA that sound incredible, granted they cost quite a bit. My runners up are Telefunken 6922 and the Tungsrams as suggested by BHK. just be sure to get tubes that are tightly matched otherwise you will suffer from volume pops. Best of luck and hop it works out!
PS Audio BHK lacking..
I bought a used PS Audio BHK preamp to replace my Rotel. It sounds thin with no depth & bass compared to my Rotel so I let it run for over an hour with no change. I'm new to tubes so I'm looking for ideas. New tubes or what?