The Palmer MONICON seems to fit the bill. It looks nice and solid and doesn't cost a fortune.
The Monicon is a clear device that is available from Thomann for just under €60. There is still the big brother Monicon L. This offers more features (such as two separately switchable inputs and outputs), but costs €100 more.
The unboxing is pretty unexciting, the Thomann package comes with a cardboard box that pretty much corresponds to the size of the monicon. Palmer also included a piece of styrofoam to completely fill out the box and instructions.
The Monicon comes in a simple black design, the metal looks really good. The wood paneling on the sides shouldn't have been necessary for my needs, but it looks high-quality and is therefore found to be in order. The workmanship is very good and the device feels heavy in the hand. There are rubber feet on the underside (next to a barcode babbler) so it doesn't just slip on the table.
On the front there is a pleasantly large attenuation control. This feels very valuable. The controller is sensibly rastered: At high volumes (low attenuation) very fine adjustments are possible, at lower volumes you can very quickly turn large areas through. That makes sense and I like it!
Next to it are two small buttons "mute" and "mono", which also do exactly what you would expect. The buttons don't quite match the quality of the large controller, but still have a good feel. You can still see two black screws on the front panel, which somewhat disturb the otherwise quite simple design.
The sound is as it should be: just like without the Monicon. I couldn't see any differences, at least in an A/B test. In this respect, everything was done correctly here. The controller runs very smoothly and there were no scratches on the brand new device. Whether something is still happening there can probably only be said after a certain amount of time.
The Monicon is the perfect device for my purpose. At 100€ it is quite cheap and can do exactly what I want. From my side, therefore, a clear purchase recommendation!