I am one who feels,in your case the sub option is a very good idea!
I've heard your speaker alot,and have definitely heard good bass,but with 100 wpc push pull amps.In a rather large room,if one can believe it.
I think,if you like your choice of electronics(a damn good one,btw)a well integrated sub(with built in amp) can be your ticket!!
I had a similar situation with my Avalon Ascent Mk-II speakers...
This is an amazing speaker(by any standard,assuming the room volume is correct).I had built a dedicated room just for it to breath,but wanted to allow for the amazing staging,the speaker can give.
These are a sealed design,with 110 lbs of external crossovers.I had them out about eight feet into the room(from back wall),which is where Avalon felt they would be optimum.Yes,I could "do" that location,and loved it,but my lowest freq(under forty hz)took a "slight" hit!No big deal,but...with the plethora of today's quite good subs(at all price ranges,and don't believe the naysayers)I felt I could get it all.
I DID!!!...but it also took me a long time to properly voice the two to perfection(the Ascents are bullet fast in mid bass,not to mention amazing resolution/tonality in mids/highs).One reason it takes so long is because once you "go sub" you cannot help but to lose yourself in BIG bass!!Alot of it,and it will ultimately(guaranteed)have to be re-voiced,once you get over the newness of so much low freq power,and ultimately decide to "go for accuracy"!You'll do that too.
IT WILL DEFINITELY TAKE SOME TIME!
I added a REL Stentor,and though ALL my friends were against this,it has worked out marvelously.
No hint of discontinuity,basically because my objective was to "only" add a hint if low freq weight/depth.
I cannot see any scenario where you should get rid of such superb electronics,that you already "know" works well with the speaker,to attempt to control the woofer better.
IMO,you'd be taking a chance,and could easily lose the magic the Shindo brings(it has a reputation for a good reason).
There are many options,in both size and price,for you to consider.One thing for sure is that you won't go through the hassle of "selling" and "buying" and ultimately taking a chance with a fabulous amp(like the Shindo).
It's far easier to actually borrow a smaller sub,to play around with,for awhile.There's some "killer" ones out there!Then you'll have a better idea of where you stand.
Good luck.