Baldy said:The simple answer is turning up the volume in the machine also increases the effect from emi etc while boosting the signal outside the machine doesn't....Not convinced about that..I would think the balance of EMI and detector noise would be the same..When you turn down the volume of your detector you turn down both EMI and detector..and boosting from another amplified source would just turn them both up again in the same ratio to each other the Amplifier is not a EMI filter..
You are partially correct. Most boosters will lift all the levels but the advantage is they can lift faint signals higher than the machine is capable of doing. If you have your machine running reasonably quiet, faint deep targets are easily missed. So a booster will help in this situation.
Some boosters though also enhance the signal and put an edge on faint broad targets making them easier to hear and harder to miss.