Vintage worth the time to explore?

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.

Why not open back headphones like Grado's for when you want to play it loud? If you don't want to try that, And I also second:

1) Not buying anything older then 10 years. 

2) Looking for a system with a loudness button/control, or at the least tone controls.