The OP has made his choice, but for the benefit of other readers:
The main "Under $400" receiver thread is here:http://www.avsforum.com/avs-vb/showthread.php?t=1092513
Prospective HK 254 purchasers should first look at it's offical thread here:http://www.avsforum.com/avs-vb/showthread.php?t=1043407
A competitive problem with the Onkyo RC160 and the 607 is that they both use Audyssey 2EQ, a minimal version of Audyssey that does not EQ the sub channel at all -- that's the most important channel to EQ in most rooms.
All of Denon's '09 lineup including the AVR-590 use Audyssey MultEQ at a minimum. The bottom line is that unless you are installing the receiver in a very well bass treated room (unlikely in this price class) or are not using a sub at all, the Denon's are going to sound better.