Excellent choices you have there.
Can you get your deposit back on the Mistral? They are great, sound wonderful and if you can not get your money back stick with the Mistral, it is NOT a mistake and could please you for a long time.
The Mac 6900 is a great piece and was the top of the solid state line until the 7000 came out recently. The 2275 is a whole different animal and while it requires a little more careful speaker selection, it is subjectively the best sounding Mac integrated between the 2 you are considering (not too tubey, just right). The Quads you are considering will work well with any of your 3 integrateds currently being considered.
The 2275 is basically a 220 pre amp feature set combined with the 275 amp feature set and is really quite nice, I have owned one in the past. I do not however believe it is as good as a 220 combined with a 275. If budget is not a constraint, you might consider that combo.