Vintage worth the time to explore?

Playing louder has at least 2 affects. For one our hearing response varies in the bass and treble with loudness. Both roll off earlier at low levels. Flat is around 100 dB if I recall correctly and that's too dam loud and can cause ear damage. Fortunately most software is engineered to sound good at lower levels. But mastering varies and the correct level can vary from recording to recording.

Another problem can be dynamic linearity(my handle). This isn't the ability to play loud cleanly(although that's a part of it). It's the ability to change levels accurately from micro to medium to macro, to not compress level changes. Unfortunately many recordings compress level changes. And the affect of this compression is most at low levels and playing loudly ameliorates this affect. Many speakers are guilty of dynamic compression. For lower level listening lack of compression is essential to sort of sounding live. Picking dynamic speakers is essential.

As others have noted, all gear sounds better when played louder, within their performance envelope.  Many will scorn their use, but if your Marantz has a loudness button, give it a try.  If you're satisfied with the result, then don't worry for now.  You can revisit this choice when your children are older.

Also, if you have heard gear played at your desired volume range and with the sound you seek, please let us know the details.

My personal take - if I could only play my stereo quietly, I'd probably own Magneplanars.  If you've only heard boxes, you should definitely check out Maggies or electrostats.

If I’m reading your post correctly, you are driving 4 speakers with one integrated amp, correct? That’s a big ask for most integrated amps. If lower volume/increased  lower frequency response is desired, perhaps try a set of Marantz MA-500 amps. They are available used on the regular. They are vintage at a lower cost. If memory serves, they can be daisy chained. Though I’m not sure your integrated has pre outs. This would be a lower cost way to see if the lower volume/increased lower freq response is doable. Without breaking the bank.

Add a pair of subs. Makes all the difference in the world at all volume levels. 

...and you can play this, so's not to strain anything.... ;)

https://www.youtube.com/watch?v=cRIfsFefatg&list=RDMMbhQRS6iy23w&index=19

😏