So I'm about to replace my senn C's and I was curious if either the A or B range generally had more open frequency's in the US and other countries. I do some occasional travelling to other continents but mostly for US.

Sennheiser A, B and C may be inappropriate or even illegal in other countries. Always check local regulations before transmitting...

Here in the US, a little research might provide a better answer to the A/B coin flip. Try this site...

http://74.205.3.123/ff/

It will find open frequencies in a given city. Note that until Feb. 17, many stations are occupying TWO channels, one for digital the other for analog. Hopefully when the analogs go dark the congested airwaves will offer more spaces for the many wireless mic operators to utilize.

Also note that this site will only list TV broadcasters, there could be many other unlisted transmissions as well. It is always prudent to use a receiver to find open channels before beginning a transmission.

For what it's worth, I have both A and B units and travel the US extensively. I always manage to find open channels to use.

Greg, that's a great source. From what I can tell there are no vacant B channels in my area so thanks. A it is.