Vintage worth the time to explore?

Well, human hearing frequency response prefers louder in order to flesh out dynamic contrasts. If tubes are out of the question with kids, consider the Rotel RA840(BX) range. Also, try solid core UPOCC copper wire in Teflon speaker wire and interconnect.

My thought is that one is better off exploring the used market for gear less than 10 years old. Most bang for the buck.

There are amps at the same price point as the Marantz  that can bring greater dynamics to the sound.  Loss of warmth and color? Maybe your speakers are the  problem.

The only vintage gear I would consider is point to point wired tube amplification from the 60s, such as the Dynaco ST70. 

Could be your room. It  will determine half of what you hear.

It could very well be the room. The ceilings are a good 20' tall in the living room, but I've done my best to fill the space with absorbing objects.

Any suggestions upgrading from the Marantz?

If you really want warm sound at any volume, you should be looking at the 6L6 or EL34 based vintage tube gear. Heathkits, Eicos, Marantz 8, Dynacos. They obviously need to have been restored for safe use. But the good news is these point-to-point tube amps are generally easier to maintain in the long run.