i dont think you actually put in 300 watt to be fair. 500hz is there most efficient region in frequency +- there stated SPL is measured @500hz for this reason.
6 ft is almost 2 meters so lets say there spec at 1 meter - 6 dB is the spl you get at 6ft. (not entirely true since its a small line source so it might be minus 4 or 5 dB)
you will have around 80 dB with 2 watt.... (2 watts since they state it at 2.85 volt witch is 2 watts into 4 ohm) 4 watt would be 83dB , 8 watt would be 86dB already. so if you would push 300 watt... you most certainly should be able to reach beyond 85 dB. i know there efficiency might even be lower but in any case 300 watt... would go far beyond 85dB, and a great change to melt the mylar foil or loosen the glue used to attach the aluminium foils to the mylar.
maybe the preamp does not put out enough voltage to utilize the maximum the power amplifier can deliver ?
My experience is with MMGI's.
150W will not do it, I had ~180W and replaced it with ~ 500W Class D which was a great move, opened up the speaker and they really sounded good. I ended up running them with a Sub and relieving them of everything below ~75 Hz. I also found that even with all that power, the speakers will only play so loud and no louder.
i think after a while the tension of the foil iself becomes like a soft xmax/ limiter compressor. that tension is insane and i cant think of a piece of wire/foil trace pulling hard enough on it after a while. having them do not do low end helps allot distortion wise ! good call.
150 quality watts will more than enough. I'm currently running a class-A amplifier with the LRS. My amp has the curcuit design close to below link but 60W in pure class-A. It makes the LRS sound great without any problem at all.
https://www.diyaudio.com/community/threads/f4-beast-builders.300233/
https://www.diyaudio.com/community/threads/f4-beast-builders.300233/