Vintage worth the time to explore?

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

😏

If you get something from the 70's what might be helpful for you is most stuff had a "loudness" switch that will give you that warmth at low volumes. The marantz you mentioned probably has one.