@500homeruns aka Steve : I slogged thru the M12 manual and didn’t see a high pass filter, settings for it and IMO a phase accurate filter ( after all you did buy a time and phase accurate speaker in the Treo - congrats btw, you should consider joing tge vandersteen owners forum on the Vandy site. Not everyone there owns Vandy subs. There are more than a few Treo owners w other subs…well integrated… )
First the Treo in most rooms are flat to 40 hz. You can implement the low pass filter in the M12 if desired. Having the sub play higher than 40 hz will create some swarm effect and smooth in room response. Obviously the sub three has 11 bands of EQ below 120 hz and is not powered by a garden variety plate amp. I wouldn’t discount 3 x 8” and a power factor corrected feed forward optimized amp…but hey the do cost $ for a made in USA sub/amp. You may want to borrow a M5 high pass filter and see what that sounds like at a range of settings. Mid bass slam and midrange clarity are typical improvement as load comes off the main amp. You can then either buy a set of filters OR implement them inside your Rogue power amps.
Have fun, let me know if i can help.
Jim