I have always read that one should prune back the butterfly bushes late February to early March. I have done this for the 20 years we've lived in this house and they always grow huge.
That said... my husband hates these bushes. Mostly because I had a few that grew outside their boundaries and he hated mowing under them. So last year I actually cut them back twice. I had a late start because my mom passed away in April, and I had to drive the hour distance to the hospital every day, so I didn't have any time for gardening. So the first cutback was late April. Then I cut them back once again in late May, after they started getting long stems. They did flower
just a bit later than usual, but they were full with lots of blooms.
This year, I cut them back late February during a warm spell, and lower than usual. They are now filling out nicely. I will probably do it again in a week or so just to keep them in check again.
I did lose 2 bushes that were in the center of 9. I have a garden area just outside my walk-out basement doors and these bushes give nice privacy from the neighbors. The center ones were probably the oldest, plus we had a lot of ice and snow this past winter, not to mention Hurricane Sandy.
Good luck with your bushes. Some of your wood could be dead - I had to trim out some of mine. But as long as there are green stems coming out, the plant is still alive. I do the scratch test to see what is dead and what is alive - scratch your fingernail or a shovel edge along the stem or trunk. If you do not see green under the outer layer, then that part is probably dead.