Moving into an apartment with wood joist floors - worried about neighbors hearing

Several things come to mind here:

Very good tips on sound isolation/absorption.  Keep in mind that powerful low frequency sound waves are very long and can penetrate feet (yes, FEET) of sound absorption.  But,every bit helps.

If you love your Kivas, rolling off the bass via tone controls/EQ will accomplish the same goals as a bookshelf speaker in bass reduction without changing speakers.  Yes, you will not get the performance you paid for, but you'll get 90+ percent of it.

Using a meter to measure sound in your neighbor's space may get you technically, and legally(?) there, but sound pollution is a 'thing" and it's not cool.  Your neighbors have made an investment in their spaces and desire a living environment that is theirs -- not yours. A rule of thumb: if your neighbors can hear your system, it's too loud.  Sorry to be heavy-handed here, but I have first-hand experience with this on the "receiving" end.  Simple reading, polite conversation, and enjoying one's own personal music is compromised. 

This a bit of a tangent, on neighbor relations, and sappy but I’ma tell it anyway. When I bought my current place 25 years ago my neighbor marched right over , 1st day, extended his hand and introduced himself. He was retired and proly in his 70’s. My house was a bit of a fixer upper and the grass was hammered due to derelict sprinkler system. Neighbor’s lawn was well kept. After he saw me working on the sprinkler and the rest of the house, he came over and offered to help. Both front and back control valve manifolds were trashed. While I was at work he went to a pipe fitter and had copper manifold sweated up and installed them, never ask for dime. That forged the foundation of a friendship with a family that lasted until his passing. Since then I have repaired heads, lines and valves over the years but I have never had to touch that copper manifold and we get zub zero here in the Rockies. So every late fall when I winterize the system and I look at that manifold all incrusted with grime and dirt , I think of him.

I like the idea of using an equalizer, when necessary, to roll off bass.  You can use it during listening sessions during the evening, particularly at late hours, and use less or no equalization at other time, on weekends, etc.  It is a nice adaptive solution that will allow you to go wild on Saturday afternoons yet reduce noise transmission at other times.  In any case, rug and pad covering of the largest amount of floor possible will help a lot, as will any coverage of the walls with damping.  

The ultimate solution, one that will generally involve ownership of the space, involves companies that do isolation professionally--they can use springs and dampers to float a floor above the original floor, they can add additional noise damping sheetrock to existing walls (isolated from the existing wall by damping material such as "Green Glue" adhesive), and even treat the ceiling.

There is some solid advise given, and some a bit drastic or too much too soon.
I had a loft studio space in an old textile building in MA years ago, so I have some experience. Keep your speakers, do all the normal furnishings you are thinking: rugs, plants, things to break up the directed sound waves, and a couple ACTUAL bass traps to start.  Like other mentioned, go introduce yourself to your neighbors, take some cookies or something from a great local place.  DO NOT MENTION YOUR SPEAKERS AT FIRST VISIT.  I had issues with one neighbor that I realized all started because I began my introductions with them by saying "let me know if you can hear my system": within a week, I was getting constant calls and door bangs for just the TV being on LOW volume in the background.  SO -say hi, maybe see how they have their spaces set up, maybe one of them has a great sound system too!  You can likely tell their personalities and possible issues from that first visit.

In my current home (1920 old dimensional wood floors, very close neighbors) I actually measured and found the best spot for my sub-woofer (now using great larger bookshelf speakers, not floor standers): I used a web site that will calculate what each speakers pressure is and best placement spots, as well as dead or excited points in your space.  The bookshelfs are on awesome custom narrow box tube stands with slate slabs on top of the stand, under the speaker, and they are placed on top of the floor joists (spikes). Then I put the subwoofer on a slate stone hunk as well with cut in half racket balls under the slab, also carefully placed on top of floor joist but a bit further from a wall barrier than usual.  The outcome was compromising some on extension/depth output, but things are much more articulate and accurate, and the sound doesn't extend into the basement or beyond the walls like when I first moved in.  

Townsend platforms I bet would be the best bang for your buck, then the traps and all.  I really hope you can keep the speakers.  Would love to see how your space turns out!   

Thanks all for the very thoughtful feedback. Based on all of your feedback, here’s my plan:

• Get a huge thick area rug to go under the stereo and LP. Thick rug pad under (maybe mass-loaded vinyl?).
• Decouple the speakers with Isoacoustic Gaia’s (which I already have). I’ll set these on granite slabs that I’ll have cut. The Townshend stuff looks great but by some reports performs about the same as the Gaia’s. (I will also inquire about Acoustic Field’s carbon platforms which look cool if heavy and pricey).
• I’ll use equalization to roll off the low end on the digital sources, especially TV and movies. I can do this in Roon for my music and will get something like a Dirac or one of those Behringer’s mastering EQs for the TV. Analog will not have this but hopefully that’ll be ok. 
• I'll continue to use the port plugs that Egglestonworks provided me.
• Will introduce myself to the neighbors and give them my contact info (but won’t mention the speakers) and just establish myself as responsive, reasonable and approachable.
• I’ll set it up as a pretty tight setup - not near field per se but 6-8’ triangle in the room
• Will use high-temp silicon caulk to seal up any holes around the radiator.
• I already have two large and two small bass traps, as well as 6 or 8 acoustic panels. Not sure where I’ll place these but they’ll be in the room someplace. Theoretically I could add a couple more large traps preventatively, the high ceilings mean I could stack them 3-high in the corner nearest to the speakers.
• I’ll continue to listen at 60-65dB or less

Will try with the Kivas to start. Should the speakers end up being a problem, I will then consider Magnepans or a large pair of bookshelves with limited LF.