Hypex fusion/marani Audio/Powersoft/ ICE

I have looked myself and have found multiple options. Hypex plateamps are cheap and contain all the necessary parts for the cross over. Plate amps and open baffle do not mix that well IMHO.

You can also build two three channel amps with Neurochrome Modulus 86 modules. You still need to do the cross over either yourself or buy something like one of the MiniDSP solutions. Less cheap but more DIY and easier to let is optically disappear.

A third option are Purifi amplifiers. The best of it's class and relatively expensive. They will provide bass control like no other amp and you could make an all out expensive but completely digital path with miniDSP 88D and an 8 channels dac like the Okto Research dac8 Pro.