You need a crossover unit that works with the signal level of your effects loop. I'm not familiar with your amp. I've only worked with line level gear, where any reputable brand of crossover would be fine.
A pedal designed for bi amping might be what you need, kind of pricey, but cheaper than replacing speakers when you forget yourself in the heat of a gig.
You run your effects loop 100% signal and send the "high" channel out of the crossover back to the return. The highs being over 50 or 60 Hz depending how low your rig actually can play. This cuts out all the stuff that just makes your speakers flap uselessly.
Or you go out and buy a preamp and one of the newer digital power amps that has an adjustable high pass built in. Same same effect.
You say you never go so low as I was indicating. I think you fail to understand. Each octave below halves the frequency. If you play a C on your A string the octave filter generates the fundamental of the low C on the B string, lots of it too. That's brutally punishing on speakers that haven't been EQ treated or crossed out of it. As you found out.