Hi Miguel,
Once upon a time it was easy to figure out what components would go with what because the power handling was always given in Watts RMS. These days, there's "music power", "peak power" nominal power" etc. etc..
Anyway, having checked the specs of your gear, the amp outputs 45 Watts RMS per channel (at 6 ohms, which is unusual. 4 or 8 are more common. I guess they're just saying that you can use either 4 or 8 ohm speakers). The speakers will handle 35 Watts RMS per channel, so you do have enough grunt in your amp to blow your speakers if you drive them at high volume for too long.
The amp has a frequency response down to 5Hz, which is inaudible, so it also matters what you're using to drive the amp. If there is a DC offset on the amp output, or there are low frequency signals (noise) generated by your signal generator that you can't even hear, you may be putting a lot more power through your speakers than you might think.
The amp in my Chladni system only puts out 15 Watts RMS, which I find is plenty. I use a 4 inch (100mm) speaker, mounted in a baffle, pics
here. I think the speaker is rated at 25Watt RMS. Hearing protection is definitely needed. With the smaller speaker you can hold it over an antinode to get better coupling whereas with a larger speaker you can cover anitnodes of two phases and not get any coupling.
Without a lot more electronics gear than you're likely to have, you're unlikley to be able to do much by way of diagnostics. So I'd say get another (smaller) speaker, with as much power handling as is economical, mount it in a baffle and try again.
P.S. I notice on Martin 's (Kiwigeo)
pics of his set-up, he looks like he has a capacitor mounted in series with the speaker. If that is what it actually is, it's not a bad idea, as it would protect your speaker from DC signals that you wouldn't hear. Perhaps we'll get a comment from Martin.