John,
The Border bows with HexIV limbs, and probably HexV, too, are very quiet. Another bow I've owned and shot is the Dale Stahl T/D recurve.
One thing I've found is the 450+ string makes for a quiet bow. Dale Stahl ships all of his bows with a 450+ string. I've also found a TS-1+ string to be even quieter still. I've shot the TS-1+ string on a variety of recurves and longbows and have found the bows to be very quiet even without silencing materials on the string.
So, there really is a two part answer to your question. Some are inherently quiet by design, the Borders and Stahl bows, for example, and nearly all can be made quiet using the proper string, properly setting brace height and using properly tuned arrows.
Bill