Thanks for all of your comments. I actually got an equalizer today and it has made a great difference in the recordings.
I would probably agree with everyone when it comes to old recordings of any other style of music BUT opera. It is completely different with opera singing. The problem lies in how the recordings were captured. They often times sang into wooden or brass tubes and were wheeled around closer and further back from the recording device while singing. Many of the overtones (formants) were never captured and many of the overtones that were captured are many times weaker than they would be/have been in person. So the equalizer has helped to boost what was weakened. Thanks again for your help.
Formants aren't exactly the same as overtones, per se.
Overtone is the technical waveform description, whereas formant is the tonal quallity derived by shape, dimension, direction, etc. that affects the type, amplitude, richness, and other subjective qualities of the sound created by those overtones.
I agree that a $100 EQ couldn't hurt a cylinder or other mechanical recording in significant ways.
Parametrics are usually better, with a three band mono often available for under $100.If you really get into the vintage recording thing, I would consider dubbing onto computer and using software such as Soundforge, Wavelab, Audition, Audacity, etc. to modify and "tweak" the recordings to your liking.
Imagine a $100 soundcard and some software allowing you to remove most of that hiss, the wow, the pops, and to adjust EQ, reverberation, and a myriad of other things.