You might want to hear Onkyo 801 and above - Rich and smooth sounding. Won receiver of the year award on Hifi magazine.
Can be hooked up to a computer by a firewire (for better sound then out of a soundcard like computer speakers). Since there is so much of MP3 music on the web, and sights like Napster, you will have a huge library of good music.
NAD also sounds musical and full - more power than Marantz. You might want to hear them to match your nice speakers.
Def Tech and Marantz is a very good combination and the Marantz line from the x300 series on have very robust power supplies so power is not a factor at all. The Onkyo 801 would sound fine with Def Tech and the internet connectivity is a good feature but the Onkyo will not have as much 5 channel power as the Marantz and certainly not as much as NAD. With Def Tech the best bets IMO are Marantz, Elite and H/K. If there are still any H/K 7200's out there for the sale prices of the last couple months I would highly suggest you check it out. Awesome power and great match with DEf Tech.