Upgrading tubes

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. 

You might try a 6DJ8 variant which was specified for the high end of its day: Tektronix, HP, German Post Office. By far the best small signal tubes I've ever heard were Telefunken 6922's, specced for HP test equipment, the stuff that put men on the moon (1962 vintage, I believe). When I was collecting, very few people understood what an HP designation really meant, and they commanded no premium at all !!!!

Thanks everyone for all the different ideas and approaches.  Will definitely mix and match these methods.

Polkalover

Absolutely, upgrade your fuses(per davidpritchard’s recommendation) and the power cord, as well. Make your changes one at a time, keeping notes on your observations and(please) let us know of the results. Have fun!  BTW: Parts Connexion has Hi-Fi Tuning's Golds and Silvers on sale. ie: (http://www.partsconnexion.com/fuse_hifituning_gold_main.html)