The F328Be are amazing, end game speakers! Congrats and glad you are loving them. However, the above graph also illustrates that, no matter how good the speakers are.....well placed subs and eq are necessary for optimal sound quality. The response below 400 Hz is an excellent illustration of why. 15 dB variations in response and rolled off below 40 Hz.
I'm a little surprised your Hsu subs aren't quite up to snuff for you but then, your speakers may just be outclassing them a bit.
Another possibility is less than optimal setup/placement. Do you have them placed symmetrically on your front stage for aesthetics?
Again, the Hsu subs are very good values for the money but maybe stepping up to a bit higher quality like Rythmik might be a better match? Just thinking out loud. I've heard lots of subs over the years(Polk PSW110, JBL550P, SVS PB12-NSD, PB2000, PSA XV15se, PSA XS30, SI HST-18, Dayton PA-460, BMS18n862, PSA S1512, Rythmik FV15HP, and JTR S2x4). Rythmik definitely has a bit of a different sound than others.
Also, thanks for posting the RLO graph..I always found it a bit confusing to visualize what setting did what. Its clear the RLO of 0 provides the most boost, while RLO -15 provides the least.
As far as cost goes....although the F328Be are currently not in budget for me, I think they are worth the money. Objectively superb speakers with very low distortion and higher SPL capability than lesser priced speakers within the lineup. It seems like the higher price legitimately gets higher quality and better performance. Compare these to the cost of high end audio jewelry speakers and they are a bargain. I think you are getting what you pay for with these, and not just prestige and/or a name brand. These would be my cost no object, end game speakers.