I am honestly astonished that anyone in the wonderful world of tubenet can still imagine that type of metal and 'rods welded on', venturi and in my opinion especially 'sound plates' - are worth even discussing except in terms of the Law of Fraud.
Please please actually read "The effect of material in brass instruments a review, Proceedings of the Institute of Acoustics 8, 91-96 (1986) (R. Smith)"
https://www.smithwatkins.com/administra" target="_blank" target="_blank" target="_blank" target="_blank" target="_blank ... A-1986.pdf"
They are all snake oil. You guys are crazy and insecure if you believe any of the blatant misrepresentations. Yes, if you FEEL better then your playing will improve. If you FEEL you have a new gadget that helps, you will play better. If you have an expensive widget then you MUST subconsciously justify spending hard-earned money on it - so again you are convinced it must help - and, sure, it does, Really - psychologically. My father-in-law was a wonderful physician who never did a minute's work outside the UK National Health system. Yes, he left very little ££ but a wonderful heritage. He always said that 60% of patients would recover with no intervention. But a pill made them feel better so they recovered better!!
Red brass, yellow brass, copper, rose copper brass, brass copper rose red - all mythology. Vibrations of the metal do happen but, as mentioned by timothy42b who is obviously, I guess, a science-based dude - are completely irrelevant to the sound. Close your eyes,
use your brains and admit it - you have been had bigly by manufacturers' sales talk. Millions of $$ for nothing. Brazen, palpable fake news.
Start the revolution!!!!!!!! We should cryogenically freeze the people spreading these false hopes, weld heavy metal rods on their foreheads, stick venturi receivers into their .... ears. put copper red brass bells around their necks and make then swallow sound plate do-das to improve their voices.
On the other hand, we could market 'tubenet 24 carat gold (coloured aluminium) sound plates encrusted with (glass) diamonds hand-crafted by technicians with hundreds of years' experience all humming Tubby the Tuba as they work. Now that would improve anyone's playing. A more refined version would have the techs humming that customer's favourite tune.
On a slightly more serious note, IMHO some real points are:
1] a pure uninterrupted air column. Note that valves are inevitable but terrible, jagged interruptions. Also, remove all dead rodents.
2] therefore also no leaks. (always have a tapered wine cork with you. Take off the last slide before the bell. Stick in the cork. Push down all the valves. Blow like mad. If it hisses don't touch it - repair it or sell it.
3] a correctly configured and shaped horn - based on makers' centuries of empirical trial and error - and perhaps a little modern computerised stuff.
4] no loose soldering which allows different parts to vibrate separately. This does NOT affect the overall sound but may rattle or fuzz.
5] the right mouthpiece, bell and bore you want for your favoured sound.
6] masses of
intelligent practice. Record yourself and listen.
7] playing
as a soloist when young in front of as many people as often as possible - charity/old people homes/ schools.
8] when older - whisky or vodka.
No one is listening out there; no one. Nothing will defeat blind faith.