Here's how your lilac can look. See how blooms are distributed low and high? that's my lilac yesterday. Spec'd for 8' but untended will go to '9.
Lesson:Lilacs bloom on previous year's branches. Prune immediately after bloom fade. Keep height down to max. scent of blooms near and below nose level. Cut blooms far back on their branches to where you want next year's bloom to occur. Plan. Pretty cool what a bit of planning can do.