Recommend a subwoofer?

Hello... Nice old Nakamichi pre-amp (610) and tuner (630) will likely be connected to SDS-440C Class D Audio Amplifier which will power a pair of Mark & Daniel Maximus-Mini+ speakers (open to thoughts on that since they are not purchased yet).

Can anyone recommend a decent subwoofer to complement this set up? The room is about 20 x 15 x 9' with an extra nook on one side.

Also, pardon my ignorance, but since it's a two channel amp, how to best connect the sub (left or right)?

Many thanks...