So it's safe to say, that a 600:150 Ohm transformer is not suitable for this circuit, right? 10k:2k5 would be better.
No. I guess that was lost in translation.
Trying again: the label '600:150' does NOT mean that the transformer can only be used at this particular impedance of 600:150 ohms. Hence, it's 'meaningless' if you're trying to figure out at which circuit impedance the transformer will work. It may very well work fine.
I'm using Haufe transformers (labeled 90436) in my builds and I don't even know, what these were originally made for. When I asked Haufe about them, they also had no clue what those were - but as long as they sound fine, they are fine to me.
When I built my first 1073 clone with MIC/LINE switch, I had Carnhill 9045 (mic level) and 9046 (line level) transformers which I accidentally swapped without noticing. I somehow even managed to wire them backwards so that at least the ratios were correct. When I looked at the labels a year later, I realized my mistake and swapped them to their correct positions. Although they're clearly designed for different purposes, I didn't notice a difference in sound before or after the swap...