Good soundproofing elements near the property boundary - recommendations?

I don't know, I'm actually not looking for confrontation.

You should also not actively confront, but just do your sport outdoors with music. Or clean windows. It doesn't have to be perceived as an offense if one doesn't know.

I found the tip about mass to be the most effective.
We once had to quiet a whirlpool pump outdoors. Its ratings were just barely acceptable, but it still disturbed the neighbors significantly.
Something heavy, like a wall, works well. Soundproof surfaces that are shaped should not be trapped between two layers; they need to be open toward the neighbor.
I would consult a soundproofing office. We received the most well-founded statements from the testing department of Bauknecht household appliances. But that was our choice because the pump was a comparable unit and we were testing devices for them anyway. Besides, we combated the sound at its source. You would practically have to enclose the sound airtight, and you can’t do that.
Good luck, it’s still possible to accomplish something.
Gabriele

I don't want any conflict with them, not even in a passive-aggressive way, because I might ruin many other "pros" just because they have one "con."

Then one can honestly only advise you to address the issue very kindly. Building some kind of soundproof wall to the neighbor can, under certain circumstances, come across as quite passive-aggressive. At least as a neighbor, I would feel pretty stupid in that situation.

And what if you ask the neighbors to position the sound source differently? Would it be possible for the neighbors to shield the bass with a small partition?

Paragraph 17 of the [Ordnungswidrigkeitengesetz]:
'(1) A person acts unlawfully who, without justified cause or to an impermissible or avoidable extent under the circumstances, causes noise that is suitable to significantly disturb the public or the neighborhood or to harm the health of another.'

The reference value is about 40 dB. At least for apartments.

I can understand your aversion to involuntarily switched-on music very well.
With soundproofing, the first question is: what bothers you the most?
Mass helps against low frequencies. Since these frequencies spread spherically, a heavy wall offers some protection – but the sound also goes over it. Look at sound barriers on highways – what they can do and what they cannot. The effect against bass in the neighbor's garden is minimal.
At higher frequencies, the propagation is more directional. Here you can either absorb (dampen) the energy or scatter it. A dense hedge suffices for that.

From how I read your attitude here, I consider a combination of measures (I wouldn't try bass since an effective wall would have to be too high) and conversation/agreement to be productive. Turning the bass down a bit and turning off the loudness is cheaper and more effective than a wall.
In case of emergency, investing in good noise-cancelling headphones is definitely worthwhile. You may not communicate with the others anymore that way, but you will certainly better control the program.

I had the same challenge in our previous townhouse development. I was annoyed when the neighbors let their boombox blare on the terrace without being there themselves. That was a habit, no ill intention, and could be minimized easily after a chat over the fence. Good luck, Max

building some kind of soundproof wall towards the neighbor can under certain circumstances

I would also give googly eyes and rate it negatively. It’s like saying "we don’t want to see you, smell you, hear you, or anything else" It could also backfire. But they probably lack this sensitivity :cool: