Thank you for your post and interest in our products.
To measure subHertz I believe that you need to sample over long period of time with out breaks in the data, if I correct (let me know if I am not), then you would require a unit that can stream. The latest units PS3000 series are all usb and capable of streaming data.
http://www.picotech.com/picoscope-3000. ... tured-PS3K
Unfortunately, the standard software that is provided with this unit is not capable of calculating subHertz, however with the driver set it is possible to write your own software.