Vintage worth the time to explore?

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. 

in my experience with Vintage there are some very good deals out there, unfortunately as time goes on many of the highly desirable pieces have gone through the roof for prices. Also, many items from the 80-90 are now coming into their own vintage vibe and there is some excellent gear from those decades. 

Every piece over 25 years old need a restoration /service to bring it back into spec. There is a reason many vintage pieces sound warm and mushy its due to old components like capacitors out of spec, bias way out of spec, etc. after a restore many of these older piece's sound quite neutral and surprisingly good. I did a Sansui set up a couple years ago and they sounded much better ( nutral) after a restoration (compete cap replacement and key known failure items). 

modern gear can get a little more detail level but not enough to warrant the silly price they charge for some of it. well, I suppose vintage gear has gone through the roof specially with some of the newer resellers charging very hi prices (I'm looking at you skyhi audio). $10k for a vintage 70's receiver get over yourselves. 

Anyway, yes vintage can be great if you do the research on what items are worth buying.  some are over hyped some are under the radar. I'll take Sansui AU111 please but not for $5000 though lol 

oh the Sansui 717 is one of the best they made in that era its better than the AU919 above it for build quality. 

Glen 

Forget amps. You want full and detailed sound at low volume? Get a used pair of the later generation Quad ESL's like the 988's or 989's. Nothing better for low-level listening!