Yes, 9 volts into the Sierra is too much and can cause problems.... The Sierra boards were designed for 6 volt input to the board to work properly. The small 6 volt gel cell batteries originally supplied with the boards are a bit hard to find and can be expensive.
The Super Caps have been the answer for me for applications where I've had limited space to work with...
Many of the older installations have been done with all the sound and R/C stuff in a tender or "Battery Car". In those cases, I've used this 6 volt battery very effectively and have one that has been in operation for 6 years...
It may seem a bit overkill, but they work very, very well and are only $8
In a Bachmann Big Hauler Box Car used for the battery car.. Negative is connected to pin #2 and positive to Pin 4 through the DPDT on/off toggle switch. The Bachmann cars have been used because the roofs are very easily removed to change batteries..