Easy 9-12Vdc to 6.3Vdc/300mA circuit?
I'd really like to post this as a comment, but I don't have the rep.  I'll try to make it a reasonable "answer".

You didn't specify your tube complement, or what the circuit does.  Would you care to share this information?

If you are flush with tubes, you can always add a second tube and put the two filaments in series across twelve volts.  6 V vs. 6.3 V won't make a discernible difference.  Maybe the second tube could be used in the circuit?