If you send one of your M4's to Danny Richie at GR Research, he will run his full battery of tests: frequency response, cumulative spectral decay---aka waterfall plot, crossover filters and parts quality, enclosure resonances, etc. He will then decide whether or not a x/o upgrade is warranted, and if so will design one. He does all this at no cost to you, just the price of shipping the speaker both ways. He has posted lots of videos on YouTube in which he shows the whole process.