From my own personal experience using the PRE-OUTS on my Harman-Kardon HK 3470 receiver, which I use in the bedroom,
You might get a little better sound quality. But, a receiver is usually compromised parts wise and sonically compared to seperates or a good integrated amplifier.
Any decent preamp in my system sounds better than the preamp section of my receiver.
Personally, I would save up and buy a decent used Adcom preamp here on audiogon, such as the GFP-565, GFP-710, or better, a GFP-750, to match up with your Adcom amplifier.
I think you would be happier with such a set-up in the long run, especially if sound quality is very important to you.
Don't get me wrong, the Harman Kardon receivers are great two channel receivers, probably among the best as far as 2 channel receivers go, but seperates sound better.
Good luck on your quest.