Kilvey Hill uses 21, 24, 27 (HD), 25, 22 & 28 (plus 34 Local). So gain for 21 to 28 are the only really relevant ones here.
Yours is 6dB-/+0.5dB vs 9.5 to 12.7 --- so only 4db at 21 and 6.2 dB at 30.
Wolfbane data was 79dBuV/m clear line of sight and 'set top aerial' on your AVForums thread:
Although you state there's a 50m away terrain 'obstruction' which would reduce signal a bit and possibly diffract to give 'standing waves' where aerial positioning can be critical.
You are so inconsistent !!
Have you tuned the TV in with the Humax by-passed and THEN used the signal meter (is it then greyed out despite being able to watch pictures from the transmitter)?
NB one always needs to be in TV viewing mode to use UHF tuning and signal metering. It will be unavailable if on an external input, for example.
You still have not provided the aerial to Humax with amp by-passed signal numbers.
Nor have you tried the Full output from the D-A and given numbers from the Humax.
Both those may reveal things.
RF pass-through must be ON to work, we know.
If it doesn't (and it will add probably 3dB signal amplification) then it's a connection/cable issue (as the same amp is almost certainly used by the Humax tuners). Check the output male plug on the PVR for damage as well as the socket (or joiner) on the connecting lead.
Another fix would be to run a second cable from the D-A, and feed the TV separately from the PVR if that is reasonably easy to do.