The first thing that I've noticed is the use of MDF... and I'm not trying to get into a debate about that. I just know there are better materials that resonate less and I'm open to using those. Again, I prefer suggestions without regards for budget. Of course everyone has budget but I'd rather not have that be a constricting factor right now.
Actually, MDF is as resonance free as any sheet good material gets. As slipperybidness said, it has the least resonance problems of all wood products, it won't warp with changes in humidity, and it makes the best substrate for applying veneer. It is the cabinet material of choice of virtually all speaker builders, regardless of price.
What other material did you have in mind? Bamboo, is no better than MDF.
You can also use ¾" void-free plywood (it costs more, but makes less dust when you cut it), or any other ¾" thick material you like. I doubt if spending more for an unusual cabinet material will produce better results. By better results, I mean an audible difference.
Internal cross bracing is more important for reducing cabinet resonance than the material (MDF vs. ply wood) or thickness. With good cross bracing, you can easily get away with ½" thick sheet goods instead of ¾".
If you really want to spend more, I would recommend
Salk Signature Sound. The SoundScape 8 speakers begin at $8000 per pair for his standard finishes. Salk is a custom builder, and will build what ever you wish. Jim Salk sells speakers designed by the same person who designed the ER18 MTM. They share a similar sound quality.
For what it's worth, the ER18 MTM design was intended to be a less expensive DIY alternative to the
Salk Veracity HT2-TL speakers. And that intention succeeded.
I'll look at the statements soon.
The Statements by Jim Holtz are also an excellent DIY design. Be aware that their cabinets are enormous.
I still think a pair of ER18 MTMs, combined with a subwoofer or two (of your choice), will provide the best sound available.