is there a similar tapped horn to Cubo12 that is arranged so as to have less dips and thus smoother for high xover application?
Tapped horn no, is the nature of tapped horn to have the driver offset, so I think this models is only for subwoofer and kickbin, but there are many BP6P that does not present this issue and will present high SPL output as TH. that is BP6S.
The back chamber is like all vented box, and the front chamber the driver should not be offset, even for models with offset front chamber, there is a way to remove this offset.
Check below, the models:
Altec817,
Splanar,
Whorn,
WMX,
WLX will not have the issue once the driver are front loaded, so no dips. The models
BP6P5,
BP6P9 could be a good candidate to remove the offset and fix the issue too if you need wider bandwidth.
https://freeloudspeakerplan.rf.gd
Note: Some vent port could have the design improved to reduce port noise / compression.