What is the budget for the amp? You can find good integrated amps with HT bypass and bass management for not too much $$. I would have to do some digging to get you make/model info, but they are out there.

Otherwise, Y adapters will work fine. If your tower speakers can handle a full range signal, there should be no worries, and they may blend with the sub better.

Speaker level input on subwoofer is also a great option as mentioned above.

A DSP would give you the most adjustment flexibility for not much $$. (https://www.minidsp.com)