Sub for single-driver desktop speakers

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.

The Submini and Arche system now works fine in my home office. Complaints minor.

The Submini was first placed under my desktop, but the bass integration with the FR2 speakers (on short stands on the desktop) was not optimal. It sounds better on the desktop, firing towards me, more or less in line with the speakers. My goal is not to get the most bass, but the best bass, sounding like it comes from the speakers not the sub itself. The bass now sounds clearer, better articulated and coherent with the midtone and treble. Interesting how "small" things like sub placement can mean big improvements. Ideally the sub should probably stand in between the speakers, but my pc screen makes that impossible. So it now stands to the left of the speakers, and is maybe a bit louder to the left, but not much, provided I tune the sub volume and crossover "just so" (low-moderate) to help the system along. The sub is small and simple, but does a good job now.