What is a really good sounding low volume system??

Make sure that the sound is full and emitting all frequencies . You ask about a good system for playing at lower volunes and my answer is one that has a loudness or "contour" switch or button like what used to be seen om the integrateds of the Golden Era of Hi-Fi. Those amps used EL84s and tubes very similar to it e.g.7868.
The reason they put the loudness switch on them was that at low volumes bass is not as well represented as the other frequencies. Today we have eliminated the loudness contour but have added subwoofers so in many people set ups the loudness contour lives on. You may just want to buy a really well refurbished vintage piece. Many had headphone jacks.
In truth modern headphone amps are a lot better than the ones on the vintage pieces. So you just want to buy o a separate headphone amp anyway and not use the vintage headphone jack.
I would use smallish box speakers or highly efficient smaller floorstanders. I can't think of any beside Totem and the Klipsch Heresys I bought in Highschool and still have and don't use very often. They need better crossovers and need the drivers to be modded to sound really good but you might luck into a worked on upgraded pair, or you can always have it done for you. There are prolly better choices the Klipsch sound is not for everyone.

If you listen at or arround 70db any speaker of your choice and not too late including typical dynamic full range, you'll be OK. I live in the apartment and none complained yet of my Aerial 10's powered by sunfire amp.
On weekends I sometimes crank it to 90dB and on weekdays music is only heard till 8:30pm. After that I don't want to disturb my macaw at his sleeping time otherwise 60dB is still good till 10...10:30pm. After 9pm I use headphones only.
Think, try and maybe you don't need to go through the struggle especially if it's all wife-acceptible...

A set of High Emotion Audio Bella Twins are much more transparent than Maggies or ESLs, are very compact and can be placed about 12-18" from the rear wall.

They are available in 16 ohms which makes them an ideal load for a small tube amp (like our S-30, which is 45 watts/channel into 16 ohms). The speaker is honest to 36 Hz, so you get some idea of the bass. Efficiency is about 92 db 1 watt/1 meter.

That would be a small system with good dynamic range that would have resolution equal to the best high end systems made.

Atmasphere, Can you think of any other amplifiers that would work with the High Emotion Audio Bella Twins?

Here's my 2 cents...also living in apartments...and love to listen loud. but now i listen to softer volumes than ever before and am perfectly happy.

1. The quieter your noise floor and better your resolution (maggies are great for this)...the more you hear at lower volumes without feeling the need to turn it up.

2. If you are a bass freak (i am)...2 things. if you get an Auralex subwoofer platform under any sub (speaker?), it damps for the guy below you. It also cleans up the bass if you have wood floorboards that 'sing' with the bass. So you get cleaner bass and less room-structural vibration. Also neighbor friendly.

the other thing about my system now as i've honed it...is as its noise floor has dropped and i've optimized the bass, is that i can get a great little gut-punch at low volume in the bass. Which at 1am is a great feeling, and i am talking about volume 1 on my cj preamp.

Once i've ended up with loads of detail and a very clean gut punch at low levels...i pretty much have stopped cranking, and the neighbors have stopped knocking! ;) good luck.