I am not sure what Doc would have made of where this is going . He probably would be sick of me not having anything finished after four years if faffing about
I have many things to thank him for. The knowledge of drivers without filters, doping, removing as many components as possible, box rigidity, ignoring the flat frequency response brigade, removing excessive wadding etc etc etc.
I am so glad I went down the force cancelling bipolar route. I keep banging on about it
but when the back of the opposing drivers are rigidly connected (the more rigid the better) the reduction in cabinet resonance is remarkable. The other benefits are very worthwhile too - spl output increased by having another driver, baffle step compensation for free and without any music sapping components needed, IMD distortion reduced thru work being shared by two drivers.
I can't think of a subjective downside to the up and down firing bipolar mid bass. Twice the box volume is required, you need a specially designed stand because of the down firing mid and there is a bipolar dip in response apparently but that is only heard on axis from what I have read. Being always off axis with the up and down firing mid bass makes it a non issue, I certainly don't hear anything untoward. Most bipolar speakers have the opposing drivers front and back firing which causes all sorts of problems including having to have the speakers well away from the wall behind them.
Luckily with up and down firing 5" drivers I can still get the speakers close to the wall, even with the bipolar baffle step comp. That is very important for us with small rooms which have other uses like living. I even suspect that having the two drivers firing up and down helps to smooth out some room anomalies because of varying distance between ceiling and floor. Can't prove any of this though it's all purely subjective through actual listening and backed up by things gleaned from the web.
Getting rid of components in the chain (especially filters) has been really worthwhile musically. Don't need a padding resistor because of four tweeters wired in series (increased impedance) drops amplifier output just enough to match nicely with two mid bass. OK, I have replaced one resistor with three tweeter coils but the listening proved it was worth it.
The scariest bit was removing the speaker level high pass filter capacitor. I once tried removing it before and the sound wax awfully distorted (obviously) and I was lucky I didn't blow them. But when you move the filter cap required to the input of the power amp (passive line level filter not active
) the improvement is remarkable. A bit more jiggery pokery ended up with me removing the dc blocking filter from my cdp. Wow is all I can say about those changes.
I am set on having four tweeters firing in different directions. The reduction in distortion is VERY noticeable especially as we are dealing with a first order high pass. Many will poo poo this idea as the tweeters are not a single point source. I think the results are vastly superior to one tweeter and first order high pass filter as long as you get the tweeters as close together as possible and behind the up firing mid bass. Can't stress those two points enough.
I used to dismiss the effects of cabinet diffraction mainly because I hadn't time to investigate it and Doc wasn't too bothered about it - cubes with 90 degree corners, drivers not rebated). My experiments lead me to believe it can have a dramatic effect. Going from a cube tweeter pod to a rounded corner dice shape was a big improvement as regards imaging, soundstage and extra detail heard. So I am taking it to it's logical conclusion which is smooth spherical enclosures.
That's what these speakers are all about. I don't expect everybody will agree with what I have written but this is the way MY ears have led me.
Yes, GRP (glass reinforced plastic) would be a good alternative. Balloons aren't completely round though are they? 
At least some of the rings are smaller and can be cut from the circle left over from the biggest rings. And the tweeter spheres will use up more waste. It needs careful panning to make the best use of a sheet of baltic birch plywood.
Or tractor power take off 