Lots of ideas as to why. Gee where to start. I'll keep it simple.
Warm low power Marantz units are good but limited in the speakers they will run without turning to mud.
I don't like them with OLA Advents for an example.
That Advent is bottom heavy and power thirsty. Your Marantz is maybe warm and not a lot of power. The result is Marantz and Advent in the shower.
On the other hand your new to you 780 will drive most any power hungry sealed box speaker very loud with all the warm full tight bass and clarity most could ever want.
The 780 is not picky about speakers at all. Your Marantz is, it needs the KGs, the 780 does not.
I would suggest you try one of the following "affordable" sealed box speakers with the 780 .....
Wharfedale 60 or 70
Advent (new large or old large)
Fisher xp7b.(not xp7)
Most but not all of the larger sealed box Realistic from the 70s
Skipp the Pioneer speakers, some are great, there's a learning curve.
Same with JBL and oh the $$$$ for the ones you want.
Dynaco Dynaco and more Dynaco
KLH 17 (and other KLH but more $ like the model 5)
The 780 can run these speakers with all but the best pre/amps.
Luci............................. you got some thinking to do.