Low Volume Listening?

[awkaplan]

I didn't think you'd go for the headphones or changing your music genres. You want to solve your listening enjoyment problem and the neighbour's problem. The worst option for you is to solve the neighbour's problem and ruin your own listening enjoyment completely.

I've been thinking about isolators and you need something that stops bass transmission through the transmission of low frequency vibration. Auralex and probably some other companies make platforms for placement under subs to stop transmission of bass frequencies to the floor. The Auralex problem is called the SubDude and I use them under the subs in my TV setup. They make a similar product for musicians to place under speaker cabinets and drum sets which I think is cheaper but otherwise identical apart from appearance because they wouldn't normally live in a living room. I'd try something like that sort of product rather than smaller isolators designed to be used in a set under components. The SubDudes aren't expensive. I think your speakers are on stands so if I were going to use a platform like them and you're currently using spikes on your speaker stands, I'd leave the spikes on the stands and just place the platforms under the spikes. You should go for a platform that's big enough to take the speaker stand rather than using 2 or more smaller platforms under each speaker stand. How well they will work for you is going to depend on how well they isolate the speakers from  your floor and how much floor vibration from your room is being transmitted to the neighbour's apartment. I can't make any guarantees about how well speaker isolation is going to work for you, all I can say is that it could help with one transmission path.

The resonance in the walk in closet may not be due to vibration transmission. It's a space like a room and it's going to have acoustical room modes of it's own just like a room. It may also be acting as a Helmholtz resonator and resonating because of the sound itself rather than transmitted vibration. Isolating the speakers may fix the problems for the neighbours but it may do nothing to what's happening in the cabinet but if the neighbours can't hear what's happening in the cabinet and you can't hear it while you're listening, it's not an issue  you need to worry about.

Something to ponder definitely.  The resonance struck me because it’s actually the upstairs folks that are more temperamental/seem to be more bothered by the “noise.”  I should explore platforms. I was just a little hesitant due to the aesthetic—simple larger feet could do the trick vs. more sophisticated options.