It depends on your speakers. The preamp section on the Yamaha should be great, very comparable to exclusive preamps. The only major difference is price (the seperates cost more because many "believe" their performance is better, the cost to make them is also higher per unit because they don't make as many). Other things on preamps factor in as well, such as simplicity, certain features, warranty, and possible better customer service for the product (though I have heard Yamaha has pretty good customer service).
Take for example the Yamaha RX-V663 which you can find online for less than $400. It has preouts, just like a preamp, as well as all the latest in audio decoding that you could possibly need for any current media. If you have speakers with moderate to high efficiency with 6-8 ohm nominal impedance the onboard amplifier on the Yamaha RX-V663 would be adequate to fill all but the largest of rooms with very respectable clean output.
Of course other manufacturers offer comparable receivers to the Yamaha RX-V663. I only mention the Yamaha because A. you mentioned you are using a basic Yamaha 5.1 receiver at this time and B. I believe it represents an outstanding value given it's performance and features.
In case it's not completely obvious I don't buy into the seperates sound better than receivers perscribed nonsense. Just like all markets there is a high end market that offers no real advantage other than making the buyer feel as though they got the best they could afford. This is also true in all facets of audio, even speakers, so you must be careful when researching and ultimately shopping for new or/and especially used gear.
Speakers, room accoustics, and your media dictate 90% of your sound. The remaining 10% relies on you having adequate electronics behind it. This doesn't mean that if you spend $2000 on stereo speakers that you should spend $1000 on amplification (and certainly don't blow funds on cables). I know when I say "adequate" you must be thinking "what he's saying is it's just OK" or "it's just enough, but spending more could get me better performance,...right?". That would most commonly be untrue. Assuming you give your speakers enough clean power* the speakers will do what they do best. Basically the only matching that needs to be done is making you can meet your clean power needs.
*If your amplifier is not clipping audibly you are getting a neutral sound from your amplifier, unaltered (audibly) from the original source.