I know what you mean with the speakers, when i was going to buy a pair of 800 D3 series speakers. I listened to 800, 802 and 803 speakers, the 800 speakers
where excellent but you need the room size and equivalent electronics to drive them to get the best from them. The 800 was crossed off as my room couldn’t
accommodate them. Then I listened to the 802 D3 again due to the turbine head which is the same size as the 800 D3, they require careful placement, the dealer never
set them up correctly. I too had the same issue of focus / confusion etc as you have encountered, I think the speakers need quite a big room to breathe and focus. So they too
were crossed off. Then i listened to the 803 D3, they were just right, good focus, good midrange, good bass and generally a enjoyable speaker. I listened to various amps and ended up buying them.
"When I bought my 803 D2’s I compared them against the 804D3’s and concluded the D3’s midrange, while more detailed, was too forward and aggressive for me,"
There is a massive difference between the lesser 804, 805 in comparison to 803, 802 and 800. This is primarily in regards to the turbine head, I believe the 800 series really begins from the
803 speaker. As the midrange treble is night and day in comparison to the ones without the turbine head.
What do you mean by this-
"I now understand what others say about the Diablo being “muscular” and “controlled”."
Does it it have huge dynamics? Does it catch you out if you have the volume turned up and your listening to a quite passage then some loud music comes through? Does the music flow effortlessly?
Whats the strings like? The bass does it hit you hard like a Mike Tyson punch in the chest? Can you listen to it forever? Did you hear things you haven't heard before?
Did you get a chance to listen to the Chain?
I think it would be beneficial for you to go back for a second listen and listen through something you are kind of familiar with likes of 803 D3 speakers. There is no point you changing your speakers
to something else as you are not going to be familiar with them. You'll be even more confused!