Good choice on adding subs. Even the lower end of the “price not an issue” stuff you are looking at all lacks bass. Yeah I might play down to 20hz with room gain but it does not mean it does it with ease and texture.
I can’t recommend trying the JL Audio CR-1 crossover enough. The Rel hook up is 1980s tech… to get subs to truly mesh well you need a high pass crossover. I would also argue subs work better with full range speakers. I think it’s a best to cross them over where the bass driver drops off and the port starts to pick up. So this would be 50-70hz for almost all ported speakers.
I will never run a system without subs again (well not my main system) but I also would never run subs without a high pass. My system will let me run with and without the high pass so I have A/B tested it many times. With the crossover it sounds like one big awesome speaker. Without it, it sounds like a good speaker with a good sub…