Finally cracked the fabric off & recycled it, after 25 years.  It's just a 6" & the port is a cardboard tube.  To point up, it probably needs some protective screen.  

The very smallest desk sized subwoofers have 6" drivers nowadays.  Most have 10".  Lions once had a crappy desk subwoofer in a cube farm.  It didn't go very low.

The old man's SB-L52's had a 10" woofer in each speaker.  You don't know what you have until it's gone but we didn't have any modern audio sources in those days either.   Music came manely from tape, radio, analog TV until 2011.  2019 brought a big leap in audio processing which further enhanced it.

Besides loads of critters, the cover had solid, tapered sides so it didn't radiate as well as the uncovered speaker.

Listening the hell out of the upgraded system, the subwoofer impact depends on the music.  Some music has a lot of it & some has none.  Some music requires more subwoofer gain & some requires less.  It generally makes the sound richer below 200Hz but its full thunder is outside normal music.  Someone has to sit down & play a bass line to make it go but constantly blasting tones at 40Hz causes ear fatigue.  It requires critical listening lions have rarely done since the 90's.  There isn't as much music which gets lions excited as there was 25 years ago.

The subwoofer gain has gradually worked lower until it might be time to step down the feedback resistor.  The mane volume similarly needs to be stepped down more.  At the same time, the old man's SA-310 tuner & the HTR-5230 were rarely run over 1/4.  It seems to be a marketing tactic to tune the gains way up.

The desk height affects the subwoofer gain.  It's kind of a return to the days before single band compression.  Multiband compression may be the end game.

It's never going to be as loud as it was in childhood because lion ears can't take the loudness.  It never gets close to saturating the STA540.  It has intermittent interference, independent of volume level but dependent on speaker cable position.  The speaker cable itself seems to pick up something.  That might explain why subwoofers have integrated amplifiers.

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The next big problem is a permanent location for the amplifier.  It's going to be hard to reach on the bench.  The 10' fiber optic & the subwoofer cable make it difficult to place under the router.  It's going to entail spending.  It seems 2 TOSLINK cables can be joined with a 3D printed part, if you can find a cable locally.

If lions still didn't use the space for exercise, as it was 25 years ago, there could easily be 2 rear speakers & a subwoofer behind the seat.  There's sort of a case for modular floor transporters which automatically reposition all the gear.  As bad as modern movies are & the housing situation is, there's a case for watching the good movies with a good sound system during the daytime, when no neighbors are around.