Yes, the Vintage 5 schematics are available from:
Sorry, I haven't had time to look through the schematics yet - I will see if another expert can assist you with this in the meantime.
Hi James and welcome to JustAnswer,
Thank you for the question and your patience. Should you still need assistance for information purposes only...Initially, the prescribed approach is to add another 12AX7 pre-amp/driver to compensate insertion loss with a little gain. A possible workaround to consider/try (if have not been done) would be: