the pre/pro ued to be the best choice for high end because they could put all of their efforts into quality and features and not worry about power (amplification). The balanced preouts then sent a nice clean signal to whatever amps you wanted. Good for HT and great for 2 channel music.
However, with the advancement of technology, the single box solution is catching up. My buddy has a large dedicated theater with a marantz 7702mkii (pre/pro) with 3x150w emotiva amps running the 9 speakers. That thing FN cranks. However, as I said above, the new technology of say the sister (receiver) Denon x4500 or x6500 could probably compete up to certain listening levels because they have most of the same features, good power and all of them have the same room correction, audyssey xt32.
Now, if you want separates then go for it, there is plenty of support for you here. However, if you do not have a dedicated theater and don't have the rack space, then there are excellent single box solutions with plenty of power and features from $500 to $4k.
Some of the audiophiles here can hear the difference in dacs between Yamaha and say, Onkyo, but I cannot. I am not even sure I could tell the difference in room correction solutions, but that is one of the thngs you could keep an eye on to esnsure your room sounds as good as possible. NAD is known to have one of the best solutions available, Denon/Marantz probably next with audyssey xt32, Yammy close with YPAO rsc and then Pionkyo, as it just doesn't have the tweakability (technical term) of dirac or xt32.
All of these have preouts and have 9+ channels of power with all if the newest 4k features, etc.
Onkyo rz830 or cousin Pioneer vsx-lx503
Yammy a2070/2080 or v2085
next level, all 9+ channels
NAD 758 v3