If I remember my smaller Advents (1st generation), they did not need a sub.
Maybe so,but the OP has Baby Advent II's.
Different speaker,different specs.
IIRC the Small Advent is rated down to like 33Hz +/- 3dB.
Those Baby Advent II's are rated at something like 60Hz +/- 3dB.
That's gonna be a noticable difference.
Also,if one chooses to "compensate" for seemingly lacking bass response the natural tendency is to crank up the bass knob.
So doing that just puts an additional load on the OP's 20wpc receiver.
A subwoofer lets one move some of that low end bass reproduction to the plate amp on the sub.
That inturn lets one dial back the receivers bass knob some.
If not running it "flat" depending on the bass controls given turnover Hz.
And doing that lets the low wpc receiver concetrate on reproducing everything else above the crossover Hz.
Some subwoofers even have specific filters to control this as a feature.
With a speaker like the Baby Advent II @ 60Hz I'd probably cross the sub over at something like 75-80Hz.
That lets the sub's output overlap the natural "drop-off" of the speakers low end response.
But, if you really like thumping, maybe so.
Done right integrating a subwoofer should be seemless and wont sound "boomy" @ all.
It just fills out the lower frequency range as would a speaker with a wider frequency range.
FWIW
Bret P.