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.