Thanks for confirming the function of the Dim Slider under Listenback in the Settings... dialog.

I've read the manual from start to finish... and maybe twice or more with regard to this chapter...

I know that, and it works almost like you said, although: you have to select the +Talkback button as well for this to work.
If the +Talkback button is off, you don't hear the talkback signal over the headphones.
This fact is not mentioned in Chapter 25.8.1, in fact, the way it works, the information in the manual is incorrect.
This is what the manual says:

Why would there be a +Talkback button on the phones if this is supposed to work like the manual says without further ado?

Maybe what I wrote was not clear:

What I mean with

is that, whatever I set the Dim slider under Listenback to, the level of all signals going to Main Out (and I don't mean the Listenback signal, but all other signals than Listenback) stay the same.
So that is a bug, since the slider does not do anything.

There is a way to set the dim level of all signals to Main Out (except of course for the Listenback signal itself), when the Talkback button is pressed, namely via the the Dim Slider under Talkback.
That is not the function of this dim slider, according to the manual, which is wrong anyway, since you also have to select the +Talkback button for the Talkback signal to go anywhere, which, of course, does not exist on the main out...
So that is also part of the bug: the Dim Slider under Talkback does what the Dim Slider under Listenback should do.

The manual should be corrected as well.

So, when you get back from the shows:
- tell me if the manual is wrong;
- test the Dim Slider under Listenback and the effect this slider has on other signals to main out;
- test the Dim Slider under Talkback and the effect this slider has on other signals to main out.
Let me know if you can confirm the bug, or if this is something specific to my situation.

TIA!

I'm talking about the Phones in the Control Room section as well.
I don't remember if the +Talkback button is on by default, but that would deserve mention in the manual as well, since the +Talkback button is not mentioned at all, and essential for it to work. (I've already quoted the manual.)

So, I do not work outside the control room. I know how it works. I've read the manual. I use TotalMix FX since the FireFace 800 came out (2004?)... So I'm not a newbie. I'm just putting the UFX+ through it paces since I want to use it live to replace an old Yamaha O2R, which has some functionality that is only found in the more expensive Yamaha desks which are too big for my use.

So, I'm sorry that I gave you the impression that I work outside of the Control Room Section, but the +Talkback button might be the reason, since it is also ignored in the manual. (I'm guessing)

I know how a real mixing desk works. I'm 61 years old, and I have been touring live and worked in studios for over 30 years using mixing desks.
Apart from having studied composition, I've also been trained as a sound engineer, quite some time ago (finished in 1988).
So please, stop thinking that I am missing all kinds of obvious things, and check what I have been describing.

My headphones are not connected to Main Out. They are connected to analog out 9-10 and 11-12, which is, I believe the default. The headphone is plugged in on the front of the UFX+ on phones 1.
Phones 1 and Phones 2 reside in the Control Room section the way you described.

My studio monitors are connected to the Main Out, which is analog channel 1-2 on the UFX+, as, I believe, is the default.

I know that the phones are used by the musician in the recording room, but of course I listen to the phones in my mixing room, since I am testing.
Maybe it helps if I explain what the Listenback function is for (so we don't have to discuss that): it is for a microphone in the recording room, that you want to listen to in the mixing room (but could be put to other creative uses as well). It is hardwired to Main Out, since that is where your studio monitors are connected to. So Dim under Listenback should set the balance between all signals routed to Main Out and Listenback, like Dim under Talkback does for the headphones that are in the recording room.

So, hopefully you will try to designate a channel to Listenback, then play with the dim button under Listenback in the Settings... dialog, and see if this makes any difference to the levels of signals routed to Main Out, as described in the manual, and as is to be expected considering what it would have been used for in a studio with a mixing desk.

I am willing to make a screen recording and put a mic in my room, and send that to you, if that will convince you that I know what I am talking about and that the issue I am talking about exists in the real world.
Let me know if that helps, or anything else I can do to bring the issue to the next level, which is, considering it as a real bug report from a person who does thorough testing and knows a bit about mixing desks.
The result might be that I am really missing something. But, it is not one of the issues you mentioned. Stating that it has always worked is denying the existence of regressions in software. It does not sound like you have tested it using the software versions I'm using with a pre-Covid UFX+. I can understand that since you have the feeling that I am confused about the way things work. Hopefully that is out of the way now.

Maybe it is good to know that I'm an alpha/beta tester for some big companies as well... So I have some experience doing that.

I appreciate your help, and I realise that possibilities for miscommunication/misunderstanding via this medium exist, but I think it is time to do some real testing.
If you currently have no opportunity to test this: at the moment I am not pressed for time. So lets proceed when you are back.

Best regards,

Arthur Sauer