How old is the Bluetooth adapter? Before about 2006 Bluetooth devices had very limited bandwidth available to ensure a reliable connection, so they transmitted crummy sound in mono. Post-2006 devices are all A2DP-compatible (and I’m sure things have moved on since then) and should sound at least as good as something piped through the 3.5 mm jack.
I bought it in July 2015.
One reason I got it was that it is powered by a 5V USB power supply, making it versatile for portable use. It doesn’t just sound worse than the 3.5 mm connection on my home system, but worse in my car too. And the source is also A2DP and aptX compatible.
I wouldn’t even know what a state-of-the-art stereo system would look like these days. I have a turntable and an amp/tuner from ~1980, a cassette deck from a few years later, and a CD player from ~1990.
I’m actually thinking of getting a new system. I think I’ve fully amortized the old one. 