I would start with the 12AX7. Usually, rolling the input driver tube has little if any affect on the sound. You can try the reissue Mullard or Tung-Sol gold pins, which cost about $30 from Upscale Audio for the platinum grade (I use that Mullard in my CAT and they sound great). If they have no effect, then chances are the more expensive NOS types may not have one either, and you would have saved a lot of money. If there is an improvement, then the sky's the limit.
The 6922/6DJ8 family is technically not a direct replacement for the 6N1P -- they have a higher mutual conductance. Without the schematic I can't tell if that would be a factor but those tubes can safely be substituted for the 6N1P.
The 6922/6DJ8 family is technically not a direct replacement for the 6N1P -- they have a higher mutual conductance. Without the schematic I can't tell if that would be a factor but those tubes can safely be substituted for the 6N1P.