[Arran]

One could often see 1.5kV caps in this exact place in both Class A or Class AB amps.

It is possible that at this capacitance the caps just can easily be bought with this WV rating, but also if the load gets disconnected while the amp is working actively the voltage across the primary can reach some large number and this, as I heard, is one of the reason for the large WV rating.
This last one would make sense where the load (speaker) is plugged and can be accidentally disconnected. If the speaker is hardwired, there is no need in anything above 500V.

Mike;
  On RCA console sets from this period they were still using electrodynamic speakers, so if the speaker were accidentally disconnected there would be no B+ voltage for either the plates nor the screen grids. I'm not sure about U.S RCA sets but the Canadian built ones had a plug mounted on the back of the speaker basket that mated up with a socket on the end of the speaker cable, and the output transformer was normally mounted on the speaker so the cable would need at least four wires in a set like this, all of which had B+ running through them.
Regards
Arran