Easiest way to generate 25V speaker signal
Easiest is to use a 25V or 70V PA (public addressing) amplifier, e.g. first hit google for me. Since you seem to like Bosch, the have plenty of those too. You can surely find low-power cheap Chinese stuff. Your speaker has options for both 25V and 70V. There plenty of youtube tutorials how to hook that up, e.g. https://www.youtube.com/watch?v=M4I9TF4hVLI

Or build your own with a regular amplifier and an appropriate audio transformer. For yours, the transformer would be something like this or this (the latter has tap for 25V-5W too and is half-price of dedicated 4W one), assuming you don't want to power more speakers from the same amp/transformer. I have to warn you from my own experience[s] with similar 100V PA transformers that the labeling of the wires tends to be effing confusing and no two manufacturers seem to agree on where to put the ohms where the watts on those taps. So if you go this route, read the [usually skimpy] datasheet carefully and have a [AC] multi-meter handy.