Glad to hear that the handle boxes won't have much of an effect the sound output - especially when we're talking about such huge wavelengths.
Here is a link to the other 21" designs that I was referring to which also looks like it relies on the chamber coupled with the horn for achieving low frequencies - actually looks very similar to cubo design but just with an extra bend in the horn path (
http://speakerplanarchive.com/othorn.php
):
I will be using the cubo sub together with 6 Fane long throw 15" w-bins (see pic below)
Have been talking with someone else on speakerplans (a speaker designer based in Thailand - username 'thawach') and he was saying that if you really want the sub bass to be noticeable, as a general rule of thumb you should look be looking at making an enclosure which has a horn length of roughly twice that of the cabs above it.
I have measured the horn path of the Fane 15" long throw w-bins to be roughly 1.2m (x2) and the horn path of the Cubo 21" Sub is 1.83m.
Below I have made a sketch of what an extension to the cubo sub might look like - this should give an extension to the horn path of + .3m (ideally guess you'd want it to be nearer to .4m but this is just a sketch...)
Would be very interested if you could send me any further information regarding the horn extension experiments of the person who tried it out in the Netherlands.
Thanks again for all your help,
Thomas.