Marchand

They are respected line-level crossovers. Some models are available in tube and solid-state versions.

Note that

1) Many cross-overs incorporate compensations for baffle step, rising response with increasing frequency, and driver resonances.

2) Many cross-overs combine drivers' acoustic roll-offs with electrical transfer functions to produce the final slopes.

3) Many cross-overs get the flattest response by compensating for the phase shift cause by their acoustic center location or band-pass functions with asymmetric cross-over slopes and points.

much of which the marchand products won't accomodate.

Without measuring the transfer functions from your existing passive cross-overs you don't know enough about what's going on to do as well with an active solution.

The other thing to note here is that #3 is a compromise for the inability to implement affordable speaker level delay networks.

People monkey with asymmetries until they get acceptable response over a given vertical window.

Ideally, you'd use symmetric slopes and delay the driver with phase lead using all-pass networks.

Obviously, this requires measurement capabilities.

If the goal is to realize the gains of multi-amplification and not to embark on speaker design as a hobby, you'd be better off buying active speakers which suit your tastes or building a respected active design that's known to work well.