Even though I'm not a fan of the Big K, the 8000s are legitimately decent speakers and though I wouldn't go after them, they would definitely be an upgrade. I the the RP 600s are the most comparable Standmount to the 8000s. They both do have some flaws: I think Richie/GR Research did XO Mods for them both, ya?
(Funny how these topics group themselves... this is the 2nd or 3rd today?
)
Anyway, I think a very important determining factor is that for the Towers like the 8000s, you need to sit ~7-8' back, minimum, so the Drivers can all integrate properly. Any closer and you aren't getting the whole picture.
As I like to say, at the end of the day it is your system and your Ducats. You have to decide where the value lies for you.
But Upgrading, as you described, your Subs and Mains, will address the most glaring weaknesses in your system, just short of actually stepping up to better speakers still.
FWIW, I like both Sub options. You should be patient for the Monolith 15 if that's in the cards. It will be the better Sub by everything I've seen, though the intangibles are SVS' CS and Phone App.