I, too, owned a pair of RP600Ms and I didn’t get along with them. After watching online videos and reading rave reviews I bought a pair during the first lock-down. I run them 24/7 for 6 weeks and they did improve most noticeably, that being said, I didn’t hear anything special from them when A/B-ing the 600Ms against my Linn Kans MK1 and my Shahinian Super Elf’s. I found the 600Ms tilted up through the treble and hence tiring listening to them. I’ve since sold the RP600Ms.
Yes, the heritage speakers do need a lot of hours on them.! When I first played them stone cold they had a thin and weedy top & mid with not that much happening down below. Took weeks on a rollercoaster of good to decent sounding days - followed by days of disjointed, tuneless noise. They took about 6 weeks to stop changing - but there were some good days during the burn-in cycle.! I found the bass took the longest to arrive, there was a time during the burn-in when the bass was so lightweight if I had sat the bass driver outside the horn on the floor it couldn’t have been any worse. But, with extended burn-in, I have excellent bass from them now. L/S are very critical to positioning in that respect.
My other favourite speakers are Omni-directional and they are much easier to setup. Just pull them out into the room and away from the side walls and that’s about all one needs to do. Klipsch speakers are very different in that respect, and extremely sensitive when it comes to the setup and room positioning, relative to boundary walls, etc, etc.
I thoroughly enjoy them, I really do. But that’s just me, my take, others will not like them. But I could care less…