Speakers for low-level listening

the fletcher munson low level listening curves document that you (all humans, even at young ages) will lose low level and sparkle, from ANY speaker or live sound at low volumes, exactly what you are experiencing. 

https://en.wikipedia.org/wiki/Equal-loudness_contour

IF a speaker sounds good at low volume, it is far from flat, it will not sound good at normal listening levels, thus you need to learn how to get the low level and sparkle back.

You need to boost bass some, perhaps boost highs a bit, and, the best way is a properly implemented automatically and progressively engaged Fletcher Munson Curve. 

Your modern McIntosh does not have a 'loudness' circuit like the old ones do. You have a 4 band equalizer, default is OFF, you just need to tell your 'too smart' amp to turn the equalizer on (separate settings for each input I think), then then you need to find adjustments that work for you at your typical listening levels, with your existing or any speakers.

Then, turn volume down, adjust your tone controls and listen. Next, return the eq to flat or off when you go back to normal listening volume.

You can also pre-adjust the levels of your various inputs, so the volume of your CD Player, TT, Streamer .... are similar as you switch them. Tricky, menu, electronic settings, actually harder than the old methods IMO.

A Chase RLC-1 Remote Line Controller has built-in automatic and progressively implemented 'Loudness/Low Volume' adjustment. None at normal or louder than normal, some or more progressive bass boost as you further lower the volume.

Your unit does not have a processor or tape loop, which makes using separate processors easy and optional, however, you can plug 1 to 4 line level sources into a Chase Remote Line Controller, plug it into your amps default input, have optional use of other McIntosh inputs ....

I even use my Chase to power my Furman P-1800, so I get remote system power, remote balance, mute, volume, and if I lower the volume, it maintains involvement by boosting the bass progressively.

I like how my shop system's Yamaha CR-1040 Receiver handles it. Two Rotary Volume Controls: one Volume, one Loudness. Set Loudness to Zero, raise Volume to normal. Now, use Loudness to lower the volume, it progressively lowers/implements fletcher munson curve. Loud: Loudness back to Zero, Volume Further Up, no adjustments occur, just 'Pump Up The Volume'.

I use it thru the processor loop, one of the ways to determine you cannot tell if it is in line or not.

https://www.ebay.com/itm/196546552664?mkevt=1&mkcid=1&mkrid=711-53200-19255-0&campid=5338381866&toolid=10001&customid=f3655672-b44c-11ef-9c01-363139623032

You must have a remote, there are no controls on the unit.

https://speakerchoices.com/

I am not sure what would determine the parameters for low level listening, but Erin's Audio Corner would have clues

I would suggest adjusting your five band equalizer to see if you can get the sound you want at low level with your current speakers before purchasing anything new.  I have an MA8900 and due to my neighbors (I live in a townhome) my late night listening is often somewhere between upper 40s to mid 50s db.  It’s around midnight right now and I’m around 50db but I have tweaked the equalizer to accentuate the higher frequencies.  I’ve got a couple of subs so I actually have to turn those down a bit even at low levels.  I find it very enjoyable to listen late night.  I still get a very good soundstage and imaging and it’s so quiet everywhere else it feels louder than it is.

I have a McIntosh MA 8900 as well.  Mine is paired with the Klipsch Cornwall IVs.  I listen at low levels most nights, I haven't felt the need to mess the the EQ at all, I seem to be able to get good frequency extension without it.  

I have to second or third the headphone idea. I have a separate headphone system myself and in many respects rivals my main system… while different it is inspiring and compelling each time I listen to it. I will say it was as much effort assembling as my main system. My systems are shown under my userID.