Where to put 4 tubes on a 6 tube preamp?

It depends on the circuit design where the most influential place to put them is. Chris Johnson (founder and former president of Sonic Frontiers) now runs The Parts Connexion, and has always been very helpful with all things SFI-related . . . you might give him an email.

If you get the service manual from mad Manuel's manuals on ebay it should tell you where it matters.
BTW I understand that only a few tubes can be used in a SF. Amongst them are the Sovteks are you sure the Amperex won't become expensive toast.
I would ask Jim McShane over on the tube asylum or just search the archives.

Doing a search on the A'goN forums for "Sonic Frontiers Line 1" turned up the following threads:

http://forum.audiogon.com/cgi-bin/fr.pl?aamps&1107063556&openfrom&1&4#1

and

http://forum.audiogon.com/cgi-bin/fr.pl?aamps&1107063556

Pretty much everything you might want to know there including what positions and tubes are best.

BTW, I owned a SF Line 1 for a few years and used a variety of NOS tubes in it without any problems.