The best one I know of is the Pass Labs, but it ain’t cheap. Is cost an object? Nelson was also offering his excellent First Watt B4 ($1500 list, but discounted), but it’s now out of production. Call Reno Hi-Fi (a major Pass/First Watt retailer), they may know of a used one.
Another good maker of active (and passive) crossovers is Marchand, who offer both solid state and tube designs. The designer offers plug-in cards with different x/o frequencies and slopes (1st/2nd/3rd/4th-order).
Bryston also makes a x/o, but it is somewhat limited in flexibility.