6x4 tube differences in a Fosgate phono preamp?

I have a Fosgate phono pre-amp that uses a 6x4 rectifier tube. Much has been said about 'rolling' the preamp tubes, but there has been very little said about trying different 6x4's.
Is there anyone who owns a Fosgate, and has tried different 6x4's? If so, what do you hear?
I own Zanden DAC with a separate PSU which uses 6CA$ and 6x4...i put in Mullard 6CA4 and Sylvanias 6X4 from Brent Jessee Tubes...great guy...and yes, it made a big difference. Much tighter bass, bigger sound in a positive way...almost like a 'stronger signal'. good luck.
The best 6X4 in the original Quicksilver preamp was GE. Quicksilver updated the rectifier from 6X4 to 5AR4 because the 6X4 did not hold up well. Many considered the 6X4 version of that preamp a design flaw because of that tube, so be careful when buying early Quicksilver preamps.