Coffee first let me say that your circuit knowledge is unmatched in this thread and I have learned a lot from you. So I can't really comment on the technical aspects as you do. All I can say is what I see, with the knowledge I have, and speak from my own experiences. I have had this resistor off for about 3 years and have never had any issues and everything works as it should. Although I do seem to go through a lot PI tubes? Could that be the reason?
From my understanding, the resistor on the presence pot is only there to keep DC off the pot with a 25k pot as DC can be scratchy sounding. From my experience, removing this resistor did not make the amp sound that scratchy, more like a bit more 'raspy' sounding. I found removing this resistor is what gives that wah sound some depth and it's woolliness. Something I don't really hear from most #34 mods outside of #34.
When I first noticed that the resistor was not on the presence pot in #34, I researched this as much as I could to learn more about it. From what I could see in the pics, and then compared it to how my presence circuit was wired before removing that resistor in my amp, I seen no difference other than the absentee resistor. From all I read, it seemed to be no big deal removing this resistor other than your allowing for more DC on the pot. I found this to be a key part of #34's sound. Whether it's right circuit wise or not as you say, I'm not sure, this is how #34 appears to be set up as far as I can tell. I'm not saying what you've stated is incorrect, you are probably right. I will have to take another look at the schematic Santiago made. But if I remember, it gets cut off right there and does not show the 4k7 resistor.
As far as the pics goes, I stated them being "clear enough" to see what is what. I do have the same pics of #34's circuit from every angle as anyone else who has went through that video frame by frame. I even spent a lot of time trying to enhance them, brightening them up and sharpening them some to clear them up a little. But they are clear enough to see what is what.
Now here is another mystery that really stumps me. And that is, there is no 150k resistor on the back of the PA pot in #34, neither is there a bright cap. That little brown mark that everyone says is a resistor is actually a burn mark on the green wire, where Frank or someone hit it with the iron. Zoom in on the photo's, enhance the color and brightness, and you will see it clearly. I tried my amp without these two components and it didn't sound too good. I believe without a doubt that they are part of the mod. So why are they missing?
There is three possibilities for this I can think of. One, when they shot the video for the AFD100 vlog, they had taken the components off for whatever reason and did not get them back on in time for the video shoot. I also thought about this for the presence resistor. But it doesn't make much sense that they would be removing components unless they were trying to measure them, or something. Even then, that doesn't really seem like a good reason.
Two, it's possible that Slash had a 150k pot installed for the PA and did not need the 150k resistor anymore. But from what I can tell, the pot looks to be the original. I think a newer pot would show a slight difference in color and age when looking at the pics and would stand out somewhat. I could be wrong, but that is my thinking.
And three, which to me seems most likely, I think that maybe at some point Slash had them removed so he could have more options with setting the PA. In the early days when recording he normally set his PA between 1 and 2 (as per Adam Day). As of late, when you look at the studio track sheets for his first solo album he is now setting the PA between 5 and 7, and you see this also in pics of #34 in studio settings on the web. If you do that with the 150k on the back of the PA pot, it doesn't sound too good, and definitely not like what you hear with Slash's sound at anytime. But the tubes your using definitely can have a huge effect on this. Since he doesn't use any kind of attenuator or PPIMV with #34 he really has no need for the bright cap either. I believe these components were there in the early days, but have since been removed. Frank still continued to implement these in his #34 mods when he did them. But he also did a few things different today than how he did it in #34 from the pics I've seen of Franks more recent #34 mods.
Now here's where things seem to get cloudy. Santiago has stated that Slash told him nothing in the amp has ever been changed since he's got it. So that leaves this a mystery to me. Because without the 150k on the back of the PA pot, the amp loses a lot of it's drive, especially with the lower PA settings, and just sounded kind of flat to me. But this would be the case since that is what that resistor does, lowers the pot's value to achieve more gain at lower settings, correct?
As someone who has worked in studios and understands the recording process, the changes we hear in Slash's sound from the old days to his more current work, there is a difference in the sound of #34. But I believe that most of this is due to how it was recorded, the studio gear used during the process, the mics used, preamps, consoles, ect, and how it was mixed, all plays a part in that. But I also think that the changes in sound is also due to the tubes he used back then compared to what he uses now, the speakers he's currently using in the studio (he tends to favor the V30's now over his usual greenback cab he used mostly during the early days), and how he is biasing the amp. Then you figure in the settings he uses today compared to those that's marked on the front of the amp (there's quite a few more than the obvious big magic marker settings you can easily see). But without the 150k on the PA, none of those settings sounded too good, or should I say correct.
All in all, my mod is exactly like what you see in the pictures with the presence cap removed, except I have the 150k on the back of the PA pot, and a bright cap (that came stock and I left it there), and it sounds spot on. It's only my playing that makes the biggest difference.