I suppose it is a case of looking exactly what the system is doing. Your source (whatever that may be) is providing the basis for what you can possibly get at the end of the chain. Compromises here aren't going to be made up anywhere else. This will feed the pre-amp, which has to pass on the signal without adding anything to it (rare), or decoded from digital to analogue, depending on your system. The power amplifier or power section is then amplifying the signal it receives - any tiny negatives are going to be amplified. Then at the end of it, your speakers are going to try and make the best of what they have been given, good or bad, or anywhere inbetween. Better quality power amplifiers control the speakers better, better quality pre-amplifiers and DACs pass on a higher quality signal.
As Esra has mentioned, I would finger the weak point as being the AV receiver - they're good, and many AV receivers have greatly improved over the years, but they can still struggle to match a high quality, two channel pre-amplifier. I can't say I'm hearing my LS50s struggle, but then I suppose it depends on the type of music you listen to, and there's only so much the mid/bass driver of the LS50s are going to be able to do.