It is a 6505+ Combo, not a 6505+, they are different amps.
Before we tear into it, what tubes have you tried in it? Both the power tube and preamp tubes. Some tubes are brighter than others. We used to have endless discussions about Clasic 30 tubes. SOme people likes JJ preamp tubes, found them smooth and mellow, others thought the JJs were dull. SOme thought the Sovtek tubes were bright and defines, others found them shrill. Bottom line - the Sovteks were a little brighter than the JJs. After that it was a matter of which sound you prefer. SO you may have some preamp tubes that don;t provide the tone you prefer. Worth spending some time checking.
And the other obvious thing is speaker. But you have tested alternate speakers.
Skipping your main topic a moment, what do you find confusing after the tone stack? The signal leaves the stack, through the master volume, through the channel select relay, then down past the reverb and into the FX loop jacks. (The signal then either goes right through R70 and into the phase inverter, or if you use the loop, through the little driver circuit and the loop jacks) From there into the phase inverter.
The phase inverter itself is very different, the combo uses a split load phase inverter, while the 5150 uses a long tail pair PI. The split load requires only one triode, while the pair needs, well, a pair of them. Your extra tube in the 5150 is a cathode folloower to drive the FX loop, and another triode to recover from it. The combo manages to do without the drive cathode follower by using the small solid state circuit. The recovery triode is still there in the combo.
Circuirt specifics. If you remove R62 as a path to ground, you will have pops every time you switch channels. It is there to keep C14 charged, it is not there INSTEAD of across the other cap. You may wish to explore resistor in parallel with cap there, but leave R62.
Changing C27 from 10nf to 1 nf will roll off less highs and make the amp brighter.
R32 and R46 form a 2/1 voltage divider. Whatever comes through C12 is reduced to roughly a third of its former level. Whatever 47k/(47k + 82K) equals. The 5150 values you have added in are a almost 15/1 reduction, what comes through C12 would be reduced to 1/15 of its former self. They are shifting signal levels at that stage.
Lift R59? Look at your 5150 circuit, the two post controls are wired in parallel, both are 1 meg controls. SO that Ultra Post has a 1 meg in parallel, it happens to be the other pot. WHichever pot is selected, the other one remains in parallel. You would change that. Note in the combo, the separate post controls are in separate channels. The 5150 onbly has one channel, so it switched controls in place. Due to that difference, the combo won;t have the other pot as a parallel, so they added the resistor to take its place. You cxan try it without R59, but not to make it more 5150-like.
R50 (R102?) Well it is 2.2k in one and 2.21k in the other, in other words 2200 ohm and 2210 ohm. If we are worried about that small a difference, half a percent, then I am in the wrong game.
R12, same agrument, I can;t really see the 39k versus 37.4 k thing making much difference.
C5 and C2. They are coupling caps linking three stages together. In the 5150, a 0.002 into the middle stage and a 0.02 coming out. SO at first they limit the bottom end and have a fuller link after the stage. In the combo they make the difference, a 470pf into the stage and a 0.001 leaving. SOunds to me like it would be brighter than the 5150.