I see those speakers have a rear port. not knowing where your speakers are relative to walls/corners, they may be making more bass, but smearing things a speck.
While trying a sub, you might temporarily block the ports, just to learn something, it takes patience and time with familiar music.
the speakers will not be trying to make low bass after the crossover for the sub, so they will sound different anyway, so I would only mess with the ports AFTER I found a sub that I might keep.
My office, my old velodyne servo 1200 sub fell apart. I tried a single inexpensive sub, got lucky: not to add bass as much as to keep things from feeling a bit thin. Volume just enough, not aware of it unless I turn it off. That’s all you need to make a small system sound beyond small.
If you ever go for another preamp/integrated, I advise having pre-out main in. then you can send the primary need for power to the sub’s amp. both the amp and the speakers get a break.
