Tubes for Modulus 3A

I took delivery of a used Modulus 3a today. I have a Modulus 3 which will go to my second system upstairs. I am not happy with the sound of this preamp, it sounds a bit thin, not much bass. I replaced the tubes with new Sovtek 6922's, but I am still not there yet.

I don't want NOS tubes and want to keep the price low and I also don't want tubes that will die in a few months. Recommendations please? I want more bass!!!