I just got my Bonnet and connected it to my XU4 - awesome! Some little glitches but nothing I would complain about.
I have one thing to say about everything above - ACCOUSTICS.
My bonnet will be for a communications receiver and the speakers will be mounted in custom built acoustic boxes which will then be mounted to the panel of the radio. Some time in the future I will post those photos.
For an experiment, you can cut a hole in the back of a Dixie cup and hot glue the board to hold the speaker in the bottom of the cup. That sound is awesome!
The best type of acoustic help is a chamber. There are closed and open chambers - I like the open ones because it seems to enhance the full bass range more, where a closed chamber seems to increase the 'thump' more than the rest of the range. For a subwoofer that is great but I'm going on a tangent.
For an open chamber, make a box the face of which is a little over twice the room needed for the speaker. Mount the speaker to the panel and make the box so that it is fairly deep. Below the speaker cut another hole 25 - 50% the diameter of the speaker and affix a tube that extends almost to the back of the box, about the diameter of the tube spaced from the back. Seal that box and wow!! I've seen it done with square and round tubes. There will be an optimum depth based on the other two dimensions of your box but any depth will sound better than the speaker by itself.
Computer speaker manufacturers do things like this -
http://store.sbs4dcc.com/images/products/detail/oscthumb.gif (I have nothing to do with them just their picture is worth a thousand words.)
I have made many chambers such as this and you will be amazed at how well even a poorly built one will sound! I use sheets of plastic from the model store and regular model glue to assemble the box. An altoids tin may work nicely. I've seen some very creative chambers built into old speaker enclosures. It's an old trick that works, and you've found out why they came up with it!
The instant you 'think outside the box' you acknowledge the box and are bound by it.
My 'box' extends beyond your known universe and only God can keep me in that ...