[Sephiroth]

The three things to grasp (this was all explained to me carefully by Igni some time ago) are:

1/
Power doubles at a 3 dBmv increase in reported level

2/
The SH tuner has 16 channels from which it can tune up to 8

3/
The number of tuned channels is not relevant to the calculation.

If only one channel was coming in, then taking your 8 dBmv level, the aggregate power is 8 dBmv. If 2 channels are coming in, that doubles the power, adding 3 dBmv (not 8 dBmv); double that to 4 channels, add another 3 dBmv; and so on till you reach the 16 channels which is 2^4. So the aggregate power is 8+3+3+3=3 = 20 dBmv.

The only caveat to the above is that I'm assuming the average dBmv; individual channels vary according to amplfieir slope and frequency; hence the use of the term "average".

If the SH allowed you to tune 16 channels, it would be the same calculation.

I wish I could say Simples!