Advice from Audiogon family - best speaker $3k-$4k

What sound are you looking for exactly?  Sf and B&W are polar opposites as others have mentioned, especially the 800 series B&W.  I've owned both and love the older B&W sound, 800 series are just too bright for my liking. 

At 3-4k you can get used Sf Sonettos and Sf Cremonas, both are exceptionally smooth speakers that are a bit shy on the bass.  They wont be nearly as detailed as your 804s, and with a mac tube pre they might be a little too smooth.  Personally I found that combo a bit too smooth for enjoyment.  The higher end Olympicas are much more detailed and made for a better match for mac tube gear (I run C47 w/ mc275 monos).  

Personally I find Focal to be a great match with McIntosh tube gear.  I've heard the Focal Aria 938 w/ a MA252 and it was an excellent combo.  Like the B&W speakers, I find the Aria line to be too bright but the tube front end rolled it off just enough to be not annoying.  I currently have the Kanta 2s and extremely pleased with them paired to my mcintosh stack.  

Now if you want something completely different from the B&W speakers, try Kef.  Their R series at that price point as very neutral and produce a huge sound stage.  I've heard the R11s on a MA252 plenty of times and it's a great combo.  

You should tell us what music you're listening.

I don’t want a bright speaker.  I like warm, but not muddy.  I need bass and detail.  I don’t see adding a sub with this system. 
I am not opposed moving the 804s and getting a different speaker for the main listening room. I have always wanted a of Maggies. 

I don’t think there are any "best" speakers out there. Some may suit your room and taste better than others.

Other brands you can try is Dynaudio with Emit 50 or 30, Fyne with F502 or F501SP. Canton also has good speakers. Or any used speakers that used to cost $6k to $8k.

He's going to be pushing the $3-4k budget with any Sonus Faber floor-stander even used.

And a lot of the other high quality floor-standers mentioned above.