The Speakers: Kali Audio IN-8 (https://www.kaliaudio.com/independence)
The Preamp: Arylic S50 Pro+ (https://www.arylic.com/products/s50pro-wireless-preamplifier)
tl;dr: Worth every penny. Approximately 115,000 of them.
So taking a step back, I had my eye on the Edifier S3000MKII speakers for a long time, hoping they'd go on sale like their little brother model did last year, the Edifier S2000MKIII, which I bought and installed in the bedroom. Those speakers are absolutely fantastic and I would highly recommend them if they go on sale again (last year it was in October). Even at full price, you're getting your money's worth with them, though there are similarly priced alternatives you might want to consider. But at the $375 price point I paid for them, no question, just buy those.
While waiting for the S3000MKIIs to go on sale, they actually went up $100 in January. I got tired of waiting for a sale and just bought them at the even higher price, a hefty $1,000 at that point.
Now the sound quality from those speakers is excellent, no complaints there. Very high resolution and flat frequency curve (putting them in the "reference speaker" aka "studio monitor" category of speakers). They have a built in preamp with plenty of input options including BT5.0 aptX HD, which is a necessity if you're going to be streaming over Bluetooth. They are independently powered, both speakers plug into the wall. Also a feature I thought I'd love, the right and left speakers connect via a proprietary HD wireless link, so no cable is necessary between the two like with the S2000MKIIIs.
Two things I didn't like about those speakers, one manageable, the other a dealbreaker. First, having the preamp built-in does limit you a bit. It can't act as an independent streaming device, so you need to either keep your Android phone paired with it to play music (which turns out to be somewhat inconvenient if you're listening to them a lot), or connect an external device of some sort which diminishes the value of having the preamp built-in in the first place. This wasn't a dealbreaker, you can actually use an RPi as a very nice streaming platform with something like this: https://moodeaudio.org
The dealbreaker was that the wireless connection between the two speakers was subject to interference. Not very often, but occasionally it would glitch, and when it did, it would emit a very loud "THUMP" that would rattle my nerves and startle the cats. Apparently this is caused by the presence of other wifi signals in the house which would occasionally briefly knock out the link. I've got three mesh routers blanketing the house, and, counting the home automation stuff, probably 60 or so wifi devices all chattering constantly, so the wifi here isn't going to get any less interfere-y, and will likely get even worse in the future. So they had to go back.
This prompted me to do more research on the subject, and after that, settled on the Kali Audio IN-8 speakers and the Arylic S50 Pro+ preamp. The total cost of these was only a bit more than the Edifiers, with objectively superior performance, determined by the real audiophile nuts who get out frequency analyzers and crap to measure response curves and THD and whatnot. They are a bit more powerful too, but notably larger in size.
These speakers are full professional grade, independently powered, but have no built-in DAC or preamp or anything, which means there are only (technically three but actually) two inputs, both analog. Thus, a preamp and analog cables are required. The two analog inputs are a standard RCA jack for unbalanced analog input, and two different jack types for the same balanced analog input. Balanced analog outputs are something you rarely see in consumer-targeted audio, they are only used in studios where they occasionally need very long analog cable runs that would otherwise be subject to interference. However, for short cable runs (say, ten feet or so) there's no practical difference between the quality of balanced and unbalanced audio, so plain old RCA inputs are just fine.
The Arylic preamp is great overall. The phone app is nothing special, but functional. The device does present a browser interface on your wifi network which is decent, though it doesn't render properly at all on mobile unless you set your browser to desktop mode. I don't think I'll actually use the web interface much. The preamp does have a remote too, which is more convenient.
The Arylic has a decent range of inputs and outputs, and can act as a stand-alone streaming device for several streaming platforms, freeing up your phone.
I'm not sure if the built-in Spotify Connect is streaming at maximum quality, which is 320kbps for Spotify. Now, I could immediately hear the difference between SD and HD streaming over Bluetooth, but switching between Bluetooth HD and Spotify Connect, the difference wasn't nearly as noticeable. It does sound a bit different, but I'm not convinced that's because of bitrate. There are a lot of other parameters that could be responsible for a slight difference in sound, like the specific codec used. According to Spotify, all Spotify Connect devices always stream at the best available bitrate by default.
Also, even 320kbps is not really HD audio anyway, so if you're really concerned about that you need go with a much higher bitrate or a lossless source, which is a whole different endeavor and one I'm not really interested in pursuing right now. In any case, my ears hear very good quality from Spotify Connect running on the Arylic, so I'm just going to use that by default regardless of what the actual bitrate is.
Finally, if you're looking for this same quality of sound but don't need 320W to shake the house, a more affordable option with equal sound quality would be this Arylic preamp with Kali Audio LP-6 V2 speakers. At full price that would come to about $650 total. The speakers are also somewhat smaller so they'd fit on a good-sized desktop, and still provide 200W of power. I'd probably opt for that combo over the Edifier S2000MKIII, which is about the same price when not on sale.
Why get studio monitors if you're going to be adjusting the waveform well away from neutral anyway?
The point is that the speakers aren't modifying the sound by their design alone. This means you can start with neutral, and then tweak things like equalizers and the "deep bass" option on the Arylic to get exactly the sound you want. Whatever your ears want to hear, these speakers can provide it.
That Moode software for the RPi I linked above might look like an appealing option, and it is. I might even set one up to try it out. However, the RPi does not have a high-quality audio DAC of course, so if you want to run analog-only speakers from it like the Kalis, you need to get a separate DAC preamp. They have instructions on how to build your own, though I would not personally do that because it looks like a lot of work and probably wouldn't save you any money. There are lots of HD DAC preamps out there that would fit the bill, like this Arylic, but there are cheaper HD DACs out there that just convert digital input via USB to unbalanced RCA output, which is all you'd need.
Anyway, I'll try this setup out for a while, and hopefully post an update after a few months.