Purely from the technical side of things only as I don't give a flying flute how well the entity on 648 gets out or not as I have zero interest in its output.

What is right though are the figurers that have already been stated above in this thread ie each time you double the power you gain 3db this dose not mater which band this is on or which end of the band 3db is 3db no more no less. The lower the frequency the longer the ground wave is also correct! But again High band or low 3db is only 3db. 4kw to 10kw would border on an increase of 3.5db signal wise it would go up by approximately half of the increase from the 1kw to the 4kw gave you! To see a good noticeable signal increase it would have to run 32kw this would give a 9db increase over and above the 4kw across the board. and if you wanted that level to rise again by 3db it would have to run 64kw for another 3db then 128KW and so on! So you do the maths simple but hey watch out for the massive electricity bill putting it bluntly a move from 4 to 10kw signal wise as apposed to the extra cost just would not be worthwhile. The best way to up the signal would be to improve the erp and efficacy at the mast, both possible, practical, and no extra cost.