Amp/Preamps vs. High end AV Receivers

The only reason to use an AV receiver is to route video signals (along with audio) if you need a one component solution for 2 channel and home theater, then that is the solution. If you have no need of the video, then all that video circuitry is contaminating the audio signal. 
You post is not clear on are you using the home theater stuff? And secondly is your gear good enough that you would even notice an improvement using a dedicated two channel stereo preamp?    
Plenty of decent receivers ...     
To me, it seems your question is a 'theory' question, when, in fact, it should be only 100% what is practical in MY situation, to give me the best sound I can afford..   Owning two cheap(er) gizmos is usually not as good as one much better gizmo.                 
Plus how MUCH TIME do you spend on each format? If you watch one movie a week, vs 40 hours of music? or the reverse?

I find the amps in most AV Receivers quite lacking...

you might try moving in stages by adding a separate power amp and using your avr as a pre--almost all mass market avrs benefit from external amping. you can always get a separate pre later.

What are the speakers that you have or interested to power sufficiently?.

When you have separates, the equipment does not have to share power supplies. This reduces intermodulations and for that reason alone allows for better sound.

But there are additional benefits- frequently separates have better power supplies, better heatsinks (in the case of transistor amps), usually better quality parts and overall better circuits.