Whitestix, a good buddy of mine has the S300 and it’s pretty stunning what those relatively small speakers can do. Haven’t heard the S400’s except at Axpona a couple of years ago but I couldn’t get a real feel for them there. That being said I have had Danny Richie with GR Research upgrade the crossovers for two pairs of speakers I own. He does fantastic work and is responsible for the crossover designs in many speakers. You may shoot him an email and see if he has any offerings for the 400. I have a pair of peachtree d5’s that he upgraded which cost around $500 for new crossovers and level matching the tweeters. Same with a pair of Era D10’s. Good luck.