Installing a new 1.6 GPF toilet?  Why don't you make it a noiseless dual flush 1.6/1.0 GPF toilet?  It is based on the same concept of STS-12.  Its enhanced air chamber and release valve on top of the flapper does the trick.

Key Benefits:

• Dual flush capability makes your toilet cleaned with least amount  of water.
• User can adjust the amount of water saving by adjusting the float ball's height on the lift wire.
• Unlike existing float ball, this is a cup type and take little place and efficient in design.
• Can easily adjust the amount of water refill by sliding the float cup on the fill valve.
• Easy to install without special tools.
• Durable ABS, Rubber construction.
• Noiseless fill valve.