The booster should go where the signal is the strongest, at the source. Putting it at the end of a long run of cable will have it also boosting any interference picked up along the way.
125' isn't that far if you use quality coax and connectors. What are you using? RG59, RG6, RG11? Are the connectors crimped on properly?
RG6 with good connectors should work. RG59, particularly with those terrible twist on connectors probably won't. I doubt you are using RG11 but it may be what you need if RG6 doesn't give you a strong enough signal.