Hi,
I upgraded from the 804S to 803D a few years ago - overall a significant improvement in the areas that are critical to me, namely bass weight and treble smoothness. What I appreciate the most about the diamond tweeter is the combination of detail without edge makes for more relaxed listening with CDs which tend to be inherently a bit harsh.
Do note that the 803D is much larger than the 804S. But I also love the look of the 804/803 - a timeless design.
Also the frequency balance is different, the 803D is bit more laid back (midrange not as prominent) than the 804S. So audition if possible to decide if you like that type of sound.
The big difference I noticed between the 803D and 802D is the more open midrange of the 802D and under-damped (a bit ripe) bass response of the 802D. Personally I prefer the 803D bass balance (in context of my listening room and amplifiers).
The new 804 Diamond and 803 Diamond are apparently more open sounding, but I dont have any personal experience to compare to the 803D. I have auditioned the new 802 Diamond very open, brighter, vivid sound quality with tighter bass than the 802D/803D.
Don't automatically assume that your amps won't drive the 803D. I found the 803D easier to drive than the 804S, both because it is a bit more sensitive, and because it did not require as high volume to achieve decent bass weight.
Mark
