I'm not sure why you picked a 1500 as the best sounding, the 500, 1500 and 2500 all used the same network, receiver, and transmitter elements - I've used all three, they perform identically, the 1500 set is actually incredibly rare - touch tone was just not common until the 2500 was out.

The best speakerphone ever made (even better than modern ones) is a 4A Speakerphone.

Also in my opinion, an AE 80 with a non cohered up carbon mic will outperform a 500/1500/2500 on a short loop (sub 15kf), and can be tuned to outperform on a long. The self compensating network in the WE set is better however.

That all said, a modern Northern Telecom or Aastra analog phone will consistently outperform a 500 set, because of the electret mic in them. Carbon mics have benefits as an amplifier, but transistors rendered them obsolete in most cases.