First, I have to say that I am learning, or in other words, I have no idea what I am talking about.

But if no one else says anything....

I have a FY3224S and a 8.2 Ω resistor and I wanted to check your question.

Also, I found a calculator for internal resistance.

https://electronicbase.net/de/innenwiderstand-berechnen/#innenwiderstand-online-rechnerSo, I measure a voltage under load (8.2 Ω) of 0.448 Vrms at 3.536 Vrms source voltage (10Vpp).

That gives 56.52 Ω internal resistance according to the above calculator.

I don't know if it's correct to calculate it this way, but the result seems halfway reasonable.

However, the waveform suffers a bit at 8.2Ω.

I think that a limit is soon reached anyway.

The question is what you expect or what can be expected.

Please correct me if I am wrong.