Every room, and every system is different. My take is don't buy new stuff until you need it. Also what is the problem you are trying to solve for? Like are you missing bass, or are you missing deep bass?
Run the mains full spread, add the sub, adjust it to fit your mains. Then live with it for a few weeks, it will take several adjustments to get right. From there, figure out if you want to cross over your main speakers or not.
My HT has everything crossed over, my mains are at 60hz, subs are also at 60hz, it works well there. Plenty of bass, clean sound.
The 2ch has the mains running full, sub crossed at 40hz. Tried all the hi-pass and low-pass settings. It was just missing something. Lost body and tone on my mains. Sounded almost hollow in spots. After weeks of messing with it, went no crossover on the mains, and 40hz on the sub, the light just clicked, this is correct.
My other half felt the same way. Was listing, she walked in, looked at me, and said "this sounds great". That was all I need to hear about my settings.