After much experimentation and extensive listening/playing tests, it became obvious that 12mm plywood is a little too flexible for the larger panels of loudspeaker cabinets and bracing is necessary. You can see the diagonal braces that I added in the picture (one per side-panel); their addition was enough to tighten up the bottom-end.
The carpet underlay that you can also see in the photograph was purchased from the Carpet Underlay Shop.co.uk and has been very effective at cutting out internal reflections and further improving the “punch” of the speaker’s delivery. At a little over £2.00 (ex VAT) per square metre, it’s something of a bargain.
The sound quality is pretty astonishing; it puts a smile on my face every time I play through it. Right now I can’t get enough of it – I really must get around to posting some new sound samples.