Remember that this is a Princeton Reverb, not a Princeton. The Princeton should stay clean up to about 7 on the volume dial. The Princeton Reverb on the other hand has an additional gain stage due to the reverb section and should stay clean to 4-5 on the volume dial.
Ned,
Stiff bottom is the goal here, so if the speaker we discussed doesn't make as much of an improvement as you think it should you can;
Replace the PS capacitors with 40mf capacitors, change the rectifier tube to a 5AR4 (if it will fit) or solid state replacement, change the PS transformer and swap it with a Deluxe or Bandmaster transformer. Constantly measure the plate voltage and if the plate voltage starts to approach 430v then STOP. Do not go over 430v with a 6V6.