Putting vintage tubes in the power supply of pre

any thoughts on using vintage tubes in the power supply section of a preamp instead of the supplied new tubes..will it improve the sound..my rig is a 2 box..power supply section is separate from the ls.
Power supply tubes do not affect the sound IMO. If they work well, they do the job. I would not bother with swapping tubes in the power supply.
Spend your money on the Phono section tubes!! (if you have a phono section.)
Some of the greatest sonic benefits in this hobby are achieved via power supply mods. ie: (http://forum.audiogon.com/cgi-bin/fr.pl?aamps&1238465914&read&keyw&zzmullard+5ar4) That refers to the improvements in a power amp, in which the output stage's gain is realized as current(read the post following his rectifier replacement). Your pre's gain stages are all voltage, but the voltage that's modulated(YES- that IS the correct word) by the signal, and conveyed to the next stage, all comes from the wall outlet, via the power supply. The tubes you choose to use in your PS can have a dramatic effect on your presentation.
Power supply tubes do not affect the sound IMO

May I ask on how you formed this opinion?

The voltage in the power supply is what makes the "sound" that you hear, technically speaking. Any change there affects everything because it dictates how the circuit operates.

What am I missing?

If you disclose the brand and model of your preamp, someone at Audiogon may have direct experience or at least can look up the circuit and tubes used and make an educated guess.
I formed this opinion based on my experience. Your experience may differ.
yes..my pre is a canary 906...i would also like to know which tubes in the LS box are the followers out of the 4..canary is brutal about responding to customers....so..if anyone knows...would be great..the tubes in the power supply are 2 6v6..1 6sl7 and 1 5ar4
Just a suggestion: try a NOS Mullard 5AR4/GZ34 from the Blackburn plant, in the rectifier slot first, and listen for improvements. Then find a NOS TungSol 6SL7GT/VT229 Round Plate(from the 40's). There are a lot of good NOS 6V6 power tubes that will each give you a different flavor in your presentation. ie: If you want a bit more warmth; you might try a pair of Brimars. For a cleaner, more detailed sound; Sylvania or Ken-Rad. RCAs will be somewhere in between. Have fun, and enjoy your music.
Good grief is Rodman on target. Could not have made better suggestions myself.