Sunfire Preamp Tubes

Greeting,

I've owned a Sunfire Classic Vacuum Tube Pre-Amp for several years.  A couple years back one of the tubes needed to be replaced so, I replaced all three with the most available low quality tubes I could find. Now, I would like to find what "better" than average tubes bring to the table. At this point however, I am not interested in taking out a loan for 1960's vintage tubes, but if I can find something better than stock to start, I am happy to broaden my horizons! Any suggestions and or thoughts?

Cheers, 

Allen

Pre-Amp- Sunfire Classic Vacuum Tube Pre-Amp

Amps-      Carver M-1.0t bridged x 2

Sub-         Sunfire Sunfire True Subwoofer Super Junior