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Consider also TINA-TI (free from TI) and SIMetrix Elements (also free). In addition to pots, Tina also has switches that you can flip in real time as you're monitoring a waveform or whatever, so you can A/B circuit options on the fly. Very very useful. And most TI opamps have detailed Tina models.

Reading in Angry German's voice: AktivBuffahSplittahBendah!

The circuit diagram itself is fine, I have built it in the past and so did quite a few other people around me. It makes a great mic pre, on par with just about anything out there in the pro audio world.

Any mic pre will make more noise (hiss) when there's nothing plugged into the input. Mic pres need to have a low impedance source (on the order of 200 ohms or less) to achieve their lowest noise. Short the input XLR pins 2 and 3 together. Still have noise? I'd try troubleshooting with a clean...

Also, you have a DC-DC switching supply extremely close to the most sensitive audio path in the whole circuit: the gain loop. That really won't help with noise. Try disabling it and using an external +/-15v supply for troubleshooting.

Does it do the same thing with the input shorted or with a mic plugged in?

^ This. Sometimes you just need a low impedance Vref. This means that it's able to supply (or sink) significant current without moving up or down with the load. A simple resistive divider made with sensible values, such as 100k or 10k, can't do this.

It's safe to use the low level 1/4" output with a simple mono cable and just call it a day. Shorting the unused Channel to ground in this fashion does not harm the unit at all. Don't do this with the 1/8" headphone output though. There is a subtle difference between the L and R outputs: one...

^ there's a weird path to ground from the emitter of T1 which might explain why it works:

Well, I'll be damned. It seems to work in Tina! You should have taken the bet, you'd be up $20 :) The only way I can imagine it working is if the PNP transistor is simply used as a passive clipping device. Maybe someone more knoweledgeable could explain how the darn thing actually works??

Ok, now I'll really put it into Tina if I find some time. As drawn, it really looks to me like it shouldn't work. Will let you know.

Easy. Put the whole thing in Tina-ti and see if it works. I have $20 that says it doesn't. But what if I'm cheating and I've already done that? 😁 (I haven't)

Like cheap beer. Headache included.

My GF bought me a whole case of these beers. What in the world? Where do you even buy this? AliExpress? Tayda? No label at all. Not even "No Name" . Just says Beer/Bière.

What I don't get is how the two transistors work, seeing that they don't have a DC path from their emitters to ground. Also, the PNP seems to go between... GND and GND at both ends??? Makes no sense. It's not powered in any way.

I haven't tried this myelf, but apparently there's a way to run Tina-TI on a Mac using Wine: https://www.diyaudio.com/community/threads/how-to-run-tina-ti-on-your-mac-for-free.409104/

You can download Tina-Ti and open the txt file I attached (change its extension from .txt to .tsc). Then press CTRL-A and you'll get a frequency response plot for different pot settings. Something like this:

To prove you're not a bot 😁

There's this too. Possibly the easiest to get started with, but I have barely touched it myself: https://www.circuitlab.com/

^ I second that motion. It may be a bit hard to get started with a circuit simulator, but if you get the hang of it, it will have been time well spent. Having said this. I tried LTspice and there was absolutely no way in hell I was going to deal with that. I absolutely hate the user interface...