Thanks for your reply.
I already tried the Pasen firmware (forgot to mention that), has the same effects as teclast firmware. I found some more rebrands, like:
Dazed C53 (russian)
I also found firmware for this player
here
But, unfortunately, same effect again...
Treelogic TL-554 (russian)
Ovito M2814
Pentagram Vanquish R Wave P5115
For which firmware can be downloaded
here
(needless to say, this one also leaves me with an untouchable player :evil: )
The only rebrand/version I found that mentions something with 291 (like it says on my PCB) is
Ifoo IF-291 but no firmware to be found for this one...
My player does not have screws, it was quite a pain to open it up, but once you have one corner slightly lifted it can be 'unpopped' from the metal base by sliding a thin piece of plastic along the rim.