https://smile.amazon.com/Klipsch-R-12SW-Subwoofer/dp/B00MJ1YR8Y/ref=psdc_12097481011_t1_B0015A8Y5M

This is a sub that's slightly cheaper and matches your speakers better.

https://www.amazon.com/S-M-S-L-Infineons-Technology-Amplifier-chip%EF%BC%8CBluetooth/dp/B081DJD1G4

This is a good amp/dac to get you started. If you're willing to spend more than that, I would suggest a good used a/v receiver from Denon, Yamaha, Sony, Onkyo, or Marantz (pricey). Try to stay under 4 years old for compatibility and reliability reasons. A receiver adds more power, a lot more tuning features, a better dac, more inputs, and more channels for not a lot more money. I wouldn't spend more than $300-400 used or $600 new for a really nice one.