If, as already was written, the noiselevel rises when you insert the antenna, you don't need an pre amp.
For that you have the audio gain.
Inserting an pre amp will overload the preamp in the transciever when you get strong signals, raise your noise level too.
Too much amplification will overdrive the first mixer as well, causing other unwanted problems.
The AGC will drive the amplification down for the regulated sections, so the end result is an higher S meter read out, but will not improve the signal quality.
Only real deaf recievers need preamps, but then mostly the reciever section needs an tune up.