Seperates are considered to have better sound because amps are high power devices and preamps are not. By seperating the power supplies and circuitry of both requirements you are supposed to get less interference and distortion. In reality a good receiver can have just as low or better distortion than a amp/pre-amp combo. So it would be hard to say the 990 has better sound than the Harmon Kardon.

In my opinion if comparing a pre-amp to a high quality receiver the "better" sound you get from separates is due to the fact that dedicated amps have more power than those built into receivers. Even THX Ultra receivers (about $4,000) are rated at 130 or 170 Watts and these ratings are NOT all channels driven simultaneously. So you are probably only getting about 70 watts per channel all driven. The cheapest outlaw amp can do 100 watts all channels driven simultaneously. Get that amp and the 990 and you are in the low 2k range, well under the range of high end receivers and with much more power and flexiblity. And plenty of power gives you plenty of volume without reaching clipping or distortion levels. Even if you dont play with a lot of volume a sudden loud sound from a movie (like an explosion) can temporarily put your receiver at distortion levels or cause clipping.

Another advantage of separates is the ability to upgrade only your pre-amp instead of the whole setup.

Receivers have the advantage of not requiring interconnects between amp and pre-amp and usually having the latest features before they reach the pre-amp world.

Bugbitten makes a good point. If your reciever has pre outs you can get an amp and see what type of improvements you get. It should be noticeable.

In terms of the 990 sounding better than the 950 some people claim it sounds slightly "warmer" or more "musical". I believe they sound the same. And even if there is a "slight" difference that some people can notice it is not the "that much more" you are referring to. It is my opinion that if you are going to upgrade your 950 you will probably want to do it for the features.