Pre-powers are when you don't want to compromise on sound or upgradability.
Premium AVR's are often out of date, ie HDR & 4K, then later on 120hz. So your £2500 AVR will be in a few years a "old skool" AVR where you don't use it for video switching, because your new HDR 4K 120hz BD player won't work through the AVR. Then you have now Atms DTS:X and pretty much every year a new tech, new HDMI revision.
AVR power amplifier stages are pretty weedy until you reach around the £2000 mark, and at that point you could get superior power with budget AVR and power amplifiers- or just buy the power amplifiers you feel you need, ie a 3 channel for L/C/R where it's most needed. For example a Rotel RMB-993 THX would destroy all AVR's, that was a 200W per channel, 3 channel THX power amplifier
Seperates are more of a long term thing- power amplifiers can last 20 years, AVRs will not, so put the money in amplification once then get affordable AV pres and just replace that when it's out of date (I still use a Lexicon MC-8) it lacks HDMI and HD audio decoding but it has plenty of coaxial and optical, and Pro-Logic/DD/DTS and Logic 7 ontop of that. So for older material Logic 7 does something AVR's do not, scale mono, stereo mixes or Pro-Logic to full on 7.1 with stereo side and rears, and it sounds bloody great.
Another benefit is clean power, you know with a decent power amp, the speakers will blow up before clipping happens. With a AVR it's possible the opposite could happen, so if you spend thousands on speakers then bit more on amps is not a huge outlay- considering what happens if you blow a treble on a now obselete speaker with no available parts.
M&K are usually 4 ohm, also if they are smaller speakers, and closed box, they'll have lower efficiency, so you need more power.
The speakers I use are pretty much all 4 ohm, minus the rears which are efficient and easy to drive, and one set dips down to 2.2ohms, so for me power amps are a no brainer. I've only bought two sets of power amplfiiers in 20 years. How often would you have bought AVR for every refresh of the decoding side, but also having to buy the same quality of amps as mine every time? I'd say £2000-£2500 for every AVR, and I've changed the av pre 5 times. So that would be well over £10,000 of flagship AVR's that are now worthless.
