I am not sure on what adding the extra support timber will do to the sound. A lot, I am sure, would depend on Tannoy's original idea on rigidness for their cabinet. Did they intend to have a very rigid cabinet? Or, did they want some flex? I know Snell wanted rigid cabinets, yet Audio Note (they use Snell designs) wants flexible cabinets. I fully modified a set of Snell E II speakers with new cabinets made from Northern Birch plywood. I also replaced the stock Snell drivers with new Audio Note drivers. This should have been a possible problem because of the differences in ideas on cabinet rigidness, but it worked great. IMHO, don't over think this. If you cabinet measurements are the same as the originals, and you are going to use drivers that spec to the originals the added bracing should not be an issue. I am no expert in this area, but so far everything I have done on modifications of speakers has worked. The speaker may not sound as the original designer intended, but they still sound good. And when it gets right down to it, isn't is all about good sound?