I tried one of these out in a shop once and found the Vader one stuttered and sounded slightly dull as well. i figured it was just because it was a shop display model, but now you mention this.....
I eventually bought Anakins fx saber from episode III and it works like a dream.
i did hear some where that some of the sabers have different sounds and pitches of hums. my anakin saber does sound different to the Vader saber i played with.
try this site for an explanation:
and search for FX sabers. i think this is where it was explained.