It is generally accepted
that the metal used in the construction of the body of the saxophone,
and that of the bell, is secondary to the metal used in the neck. This
is because the saxophone is an open-pipe instrument. While a saxophone
will always take advantage of the neck in producing a note (i.e. air must
always flow the neck), a saxophone will only take advantage of the sterling
silver bell of your Silver-Sonic, for example, when you're playing low
C#, C, B and Bb.
Copper and bronze are supposed to "warm up more evenly" and produce a more mellow tone. Gold has an extremely dark sound. Silver is very bright. Brass is kinda "in between". Plastic, I have been told, is very free blowing, but has a bit of a very bright edge.
(Nickle plating is very much like silver in its acoustic properties. Note that most nickle-plated horns were marketed as a company's "step up" model, with brass being their lowest and cheapest models.)
Plating or lacquer affects a saxophone's sound to a lesser extent and it is based on how much plating is actually used. The most common platings are nickle, silver and gold (which is actually gold plate over silver -- gold does not electroplate directly to brass). Lacquer or enamel is essentially just a cover for brass or silver and probably does not affect sound much. There are some rumors in the Selmer world that removing the lacquer from your horn does make your horn easier to blow, but I don't buy it.
In any event, a horn with a solid gold neck that has heavy gold plating (like an early Conn or Buescher) will have the darkest sound. A horn with a sterling silver neck and silver plated body -- and perhaps a sterling silver bell -- will have the brightest sound.
A heavy brass horn can sometimes produce a tone as dark as a gold plated one, provided the horn uses a lot of metal for it's construction (thick walled). The prime examples would be the Selmer Mark VI and the older Martins.
Thin walled brass horns have a very tinny sound, with the Yamaha 23's being a notable exception (it's not as tinny as one would expect from the thickness of the metal) and the Holtons being the prime example.
The thickness of the metal is the primary reason why you don't want to relacquer a horn: the shop will dip the horn in a chemical solution to remove all existing lacquer and then buff the horn so the new lacquer will stick, and shaving off a bit of the metal in the process. This is the reason you can see two identical looking Mark VI's next to each other, with a sign on one that says "relacquered" and the other saying "original lacquer" and the "original lacquer" will sell for about $500 more.
There is more than one study
out there that "proves" that the material used in the construction
of a conical bore instrument has no bearing on it's sound. In practical
application, however, I can heartily disagree. In my experience, I have
played horns with many different platings and have determined a wide variety
in timbre. My wife, who hasn't played many models of saxophone, was able
to hear and feel the difference between a Selmer Mark VII and her Selmer
USA pro model.
The best example of resonators being put to good use, that I've seen, was the stock Buffet screw-in, domed metal resonators on the Super Dynaction. The SDA is a horn about as heavy (or a little more so) than the Mark VI, so the slight brightness of the resonators, balanced by a classical mouthpiece and the horn's innate darkness, creates a very warm timbre.
That Don't Affect Tone Quality: Springs and Keys
The keys are probably the most undervalued part of the horn, and they shouldn't be. They should be made of a stong, non-bendable metal, should be "loose", so you don't have to mash the key down (like the low C#, B, Bb cluster on Conns and Bundys), shouldn't click when pressed (even if you have a Martin Typewriter :) and shouldn't come in contact with another key or rod when pressed (like it does on my Amati). The keys should be an easy reach for your hand and should be logically layed out (not like the Holtons). The best setup really is what's on the Selmers, from the Balanced action until today, with Yamaha coming in as a close second.