The problem is your sub should compliment the mains, instead of fighting with it like small cat could fight huge bear - it's not going to be happy ending for the little sub
B&W 804 do have a lot of bass so I understand you lack of desire to add sub, especially yours
First thing what I would do is try to EQ the peaks in B&W 804 response (which is far from ruler flat = not very accurate - or also could said it's "coloring" the sound)
Bowers & Wilkins 804 Diamond loudspeaker Measurements | Stereophile.com
Room Measuring/EQ gets you nowhere - time to change the mains
Most likely - You don't need new AMP, you need to fix what you have (with DSP) or change speakers - that simple
See AH article -
When to Add a Power Amplifiier to an A/V Receiver | Audioholics
If he likes how them sound I don't think he should change them, I personally am not a BW fan although I admit them fat little diamond 802's tempt me, I don't know why {maybe because they remind me of danny devito}, I have heard so many better sounding speakers for much much less money...
If you like your speakers and just want something different there is room in your current setup for improvement, I have heard the difference between avrs and more direct paths, even with everything shut off they change the sound... and like I always said not always for the worse, room correction works great {for movies} but for music I want it as is, I want my electrons to add and take away NOTHING if possible, no eq, no room correction, ect, start with zero as if the band was there in the room, their gear wouldn't have room eq... I have seen many live performances, and I am searching for that sound, not talking about concerts {unless I am listening to a LIVE track}, Im taking about when we seen Bruce play in a 30x30 room sitting on a marshall amp that was turned off, or when we go to the small concerts to see artists like Amber rubarth, dave egg, jay mraz all unplugged 10 feet from us...
I spend a lot of time listening to a few artists I like to go see when ever they are within driving distance, one that never lets me down in Amber, I have flown to a few performances too...
https://www.youtube.com/watch?v=Lrk93DSMZRw
https://www.youtube.com/watch?v=_X53DogA6JM
https://www.youtube.com/watch?v=9FpvpyXgJrw {amy is good too, when she remembers the words} buy their music support the struggling musicians and more will come, and more will struggle, its a fun cycle to watch and listen to....