I built the Stage 4 Pis and love them...there is no comparsion between these and my old Klipsch Reference Series speakers.

I am building some One Pis for surround duty and will eventually build another Stages Series speaker for center channel duty.

If you want the smallest Pi speaker with the most output and probably the biggest bang for the buck then the Stage Series is an excellent choice.
If you have more money and want a small high output speaker then the Pro Series 4 Pi is the one to go with.
If you have space for a larger speaker and want to spend abit less then the Theater Series 4 Pis is a popular one.

Lots and lots of choices!