You're right the 5800+ and the 5803 are not the same., I'm sorry if I led anyone to believe that. However, the processing/sound quality, etc. are. I will try to detail the differences below- which are numerous. For pure audio, however, most the differences seem rather extraneous to me.
This is a link to a DENON document detailing the differences between the 5800/5800+/5803:
5803_vs_5800up_vs_5800
Also, some more differences are detailed below. Note that the differences betw. the 5800+ and the 5803 are not primarily the kind that affect sonic quality. Although the 5803 does have better bass management from the EXT inputs, which could be useful if you have small speakers up front. The processors, etc. are the same. The newer 5803s (actually 5803As) have Pro Logic IIx and HDCD decoding at the receiver. I have two sources capable of HDCD, so it is not very important to me, nor is Pro Logic.
1. The 5803 has a "trigger out", 5 and 12 volts, the 5800 upgrade does not.
2. The 5803's color difference (component) monitor out jacks are able to up-convert video signals and display on screen information. the 5800 upgrade does not have this.
3. The 5803 has in it's setup menu, a video input mode, which outputs the input signals from the monitor output terminal. the 5800 upgrade does not have this.
4. The 5803 has the ability for the user to setup the method of playback, of the external 1 and 2 input jacks; and also allows the user the ability to adjust the surround parameters of a component attached to those jacks. These two inputs have full Bass Management capability. the 5800's ext 1 and 2 inputs jacks do not have these capabilities.
5. The 5803 has an 0.5 db increment adjustment on its
volume control verses the 5800's 1 db increment adjustment of the volume control.
(Note: The above 5 items are from a post by Harry Grant on the Flagship Receiver & Amplifier Central- 6/11/02).
Hope this is helpful.
Regards,
Rob