audio and transmit bandwith
The PRO features three different transmit audio bandwidths (on SSB). The bandwidths are fixed, and are called narrow (2.0 KHz), middle (2.6 KHz), and wide (2.9 KHz). The width does indeed change, and is clearly detectable both by another station, and in the audio monitor. Note that the transmit audio bandwidth is independent of the receive audio bandwidth.
NOTE: In January of 2002, I learned that some amateurs had discovered that models of the 756 past a certain serial number were built with a normal/wide transmit audio bandwidth switch. The serial number of the change appears to be around 3,000. The switch is totally undocumented, and is located within the radio. The switch, S801, is on the bottom of the radio. I do not know where it is located. The swiched is marked N (normal) and W (wide). By default, it should be in the N position. If you move the switch to the other position, your transmit audio bandwidth will become wider.