Uh you guys are steering me away from doing it.
I don't see a problem with custom chambers as they are 100% reversible. No?
I'm mainly doing this as a cosmetic "upgrade." I know different woods yield differnt acoustic performance, but I assume there would be a minor change in sound signature.
I'm deciding the wood to use for my th900 at this stage of time
Custom chambers are completely reversible, and likely not even detectable. I doubt there's anything to worry about with warranty, but to each their own.
With the custom chambers, and Lawton mods, you may find an improvement in the tightness of the bass, not that there's anything wrong with stock.
If you do it, let me know (PM if you don't want to be questioned) who is doing the mods for you. I figured if I chose to do the chambers, you could sell the stock cups for a pretty penny, and probably recoup the cost of the custom cups. That said, Lawton cups for the TH900 from my first look at them was near $400 or so, and Martin Custom Audio is typically from $200-250 for his work. He won't damp the cups he makes but you can do that yourself in a half hour if you choose to.