Sub for single-driver desktop speakers

Update - my Arche linear power supply to the amp - in a separate box - has a bit of hum, and it persists even if I place the supply below the desktop. The hum is from the box itself, not from the speakers (only if I turn speaker volume way up). Maybe I got a faulty unit. Meanwhile, I have bought a Nexa remote control so it is easy to turn it on and off. Although the hum is not so intrusive when I play music, I need full silence in my home office.

What amp are you using?

I bought the FR2 speakers, amp and power supply as a package. So the amp is Arche Audio Opus D50A.

John at Arche will send me a new transformer for the power supply, to get rid of the hum problem.

I changed the DAC from Fiio X3ii to Steinberg UR22, resulting in a bit more smooth sound, with slightly more deep tone, when streaming pcm from Qobuz.

The speakers need break-in, before I can say more, like do I want a sub, or not.

Fostex PM Submini2 arrived today - very interesting - seems to cure most of the problems with the FR2. Now I get upper bass and deeper middle tone OK. Without much smear or detriment to the FR2. Remains to be seen, how well it can be integrated. FR2 mellowing a bit, over time.