Also, is it ok to use 3/4" thick instead of 5/8" and adjust the cabinet size accordingly to get the right internal volume? Or do I even need to adjust the size as all?

Finally, I would like to say thanks to Wayne's great customer service and support. I hope the speakers are just as great!

Most retail stores here don't carry Baltic Birch, so a casual purchaser would say it's hard to get here too. But wholesalers stock it. You just have to know someone in the business of cabinetmaking. They can probably get Baltic Birch for you and I think it's worth it.

As for using a thicker material, that's fine. A slight change isn't going to make much difference but it is also easy to add 1/4" to each dimension to make up for 1/8" thicker wood.