Beetle - Thanks for the reference products.
Jim did control out of band problems as you say. However, his major tool was to create mirror circuits for the relevant troublesome resonances - electronic anti-resonance circuits if you will, which cancel each problem in the frequency & phase /time domain. Most of the parts in those huge xo networks are series and parallel shaping circuits to tame the driver misbehaviors through many octaves. Beyond just having the chops to pull it off, a big problem is the parts cost, and the juggling act of what to spend to reach what performance plateau.