EDIT: Marco, I just re-read your post. It seems that you are looking for separates, not an AV receiver. In that regard, please take the following as you will.
If you are not aware, Onkyo has just announced its latest generation of receivers. I had an Onkyo receiver years ago that I loved. The sound quality was a huge step up from my run of the mill Pioneer receiver.
I'm sure other like Pioneer Elite, Yamaha, Denon, and Marantz will soon follow suit since the first Onkyos will be available this month. In fact, I spoke with a Pio Elite rep earlier this year who told me they are introducing their next line "sometime in Sept."
I'm in the market for a new HT receiver myself. As the previous poster noted, you should be mindful of the new technologies for HT. Specifically, Dolby Digital Master, DTS-HD, and of course, HDMI 1.3. The new Onkyos have all that and more. I'm going to wait to see what the others do before deciding. By the end of the year, they all should have released new lines.
More to the point of your question, I would personally rank the more well-known HT receivers lines in this order for 2ch sound quality:
1) Marantz;
2) Harmon Kardon;
3) Denon;
4) tie, Pioneer Elite and Onkyo; and
5) Yamaha.
These are among the brands I've owned or heard extensively in recent years. This is not to say, mind you, that I think Yamaha is lacking. On the contrary, I think 2ch sound quality in HT receivers has improved dramatically in recent years. In fact, I'm anxious to see what the new ones will do in this regard.
Happy upgrading...
Anthony