Calibrating the other speakers (not just the sub)

Having had great success in the past using REW with my BFD and sub, I'm now wondering about the feasibility of using REW on the other speakers in my 5.1 set up.

My room is far from the ideal room acoustic-wise. I use the YPAO on my Yamaha receiver which helps. But I'm sure that if I had the right equalizer in front of my mains and surrounds, I could do even better.

Do I understand correctly that the BFD is not ideal for doing the mid and upper frequencies? If so, what type of reasonably priced equipment/equalizer could I add in front of my AVR to handle this. Then again, this may not even be possible because perhaps it has to be tweaked after the AVR input yet before the output. IOW perhaps I will need a different AVR?

I'd love to hear your thoughts on what is possible on a reasonable budget and whether some folks are doing this and if so, with what equipment and configuration? Thanks!