Should I use a 50 ohm adapter?
You could which would give you a 25 Ohm source for the speaker but I don't believe it is necessary.
We sold a SDG810 a few years back to a chap that wanted to investigate so called healing frequencies and IIRC we never terminated the output and just drove the speaker directly.
He was an acquaintance and I think I delivered and proved it would do what he needed, which of course it did.
Far too long ago to remember the settings....you'll work it out.