I turned on "Idle Scan" for the network. At the same time, in an SSH session, I ran "sudo femon -H". This was the output:
user@host:~$ sudo femon -H
FE: Silicon Labs Si2168 (DVBC)
Problem retrieving frontend information: Operation not supported
status SCVYL | signal 0% | snr 0% | ber 0 | unc 1 | FE_HAS_LOCK
Additional note - today I installed OpenElec onto a spare Raspberry Pi, which can also install tvheadend. Doing this, plugging in my USB tuners, the channels scanned in. I had some other problems, but they may be related to the RPi platform. Underlying fact = incoming connectivity is fine, problem is the drivers / firmware on this Ubuntu (SERVER) 16.04 Xenial instance.
I cannot believe I am the only person running this version of the distro, or TVH on top of it, with these USB tuners as they are quite popular.
Should I simply copy the .fw files, and everything else works? This is what I am led to believe!
My old installation was Ubuntu 14.10 with a manually updated kernel to v3.19. Likelihood is I probably installed other components, such as "linuxtv.org" media_build stuff.