You need a RNC compressor somewhere in your signal path.
Use the "Super Nice" mode (double compression).
A cheap experimental solution is to put a small low voltage, low wattage light bulb in series with one conductor of each of your speaker leads. Choose a value that glows at levels you don't want to exceed (this is a potentially tedious and moderately expensive part of the exercise).
This is an old trick. It acts as a compressor, but it's usually hidden inside the speaker box where you can't see it. I have seen this used on fairly recent Tannoy speakers that were intended for commercial PA use, if you can believe it, as well as old guitar amps. The action is to compress approaching limits, then act as a fuse, ultimately.
If you don't really know what you are doing, DON'T DO THIS - buy the compressor!
It's only $200 and should bring you much pleasure experimenting with various settings.
Once you have established what you like in compression (how many dB of gain reduction you like, etc.) why don't you inform us?