I can recommend the following books if you are interested in learning about exhaust design:
The Scientific Design of Exhaust and Intake Systems (Engineering and Performance)
Four-stroke Performance Tuning Second Edition

Silencing a big single isn't an easy task; the noise frequency is low but penetrating. As the pressure wave moves along the inside of the silencer there are a few different methods of actually providing a reduction in noise. The most common is to have a series of perforated tubes and plates to break up the path of the pressure wave, thus reducing the volume that comes out of the silencer. This is known as a reflective silencer. If you cut open one of these silencers then they will look something like this-



That's a car silencer, but you can clearly see the tubes, plates and volume of space available for the pressure waves to move around in.

What is often referred to as a 'race' or 'straight through' silencer uses the method of absorption through a single perforated tube, wrapped with an absorbing material such as glass mat or wire wool. Like this-



They are referred to as 'straight through' as obviously the pressure wave can pass straight through the silencer, without losing as much gas speed as in the normal reflective type of silencer, thus you lose less power.
They are also very easy to make as all they are is a tube, with a perforated tube inside. You can alter the length to provide more or less silencing, and with an increase in length you will get a slight increase in back pressure.
What is more important in exhaust design is the length, bore and shape of the pipe work from the exhaust port to the silencer. The silencer is only really there to silence the machine. The changes you can make to back pressure via how much you choose to baffle or slow down the gas/pressure wave will have an effect upon the performance of the machine, as will how open or short you have the silencer, or whether you fit one at all.
You can make adjustments as well by the inclusion of cones in the exhaust system, in order to reflect the pressure waves back, or allow them to expand.

However; it is very important not to view changes in isolation, if you are wanting to have a less-restrictive silencer with a view to gaining an increase in power from your machine, then you have to anticipate that the gas speed will increase, and thus your combustion chamber will be emptied faster. Without increasing the amount of fuel, and the corresponding amount of air, entering the combustion chamber, you will have a lean mixture which will provide negative results not only in power, but also in engine longevity.
The XT660 engine has a twin port head and as such the gas speed will be higher compared to if the same engine had a single port head. This in turn means that the amount of silencing needed is greater in comparison to a single port as the frequency of the sound waves will be higher.

After experimentation with single and twin silencers of differing lengths, I believe the most efficient and balanced method of silencing the XT660, with positive results in the power stakes is a longer single silencer with an absorption baffle, placed after a shallow reverse cone megaphone.

Hope that is of some use to you?