An easy solution is a good transformer. A converter usually has a 10K input; the CMR is 15 ohm output, right? So something like the Jensen JT-13K7A does +14dB of almost noiseless gain. It's not cheap though!
Transformers are often a useful addition to the front end of an amplifier, but you have to be careful about getting too much gain out of your transformer. It really depends on where the noise in your signal path is coming from. Your mic can be modeled as a signal voltage source in series with a noise voltage source and a resistor whose value is the output resistance of your mic. The noise voltage source is to model the acoustic noise due to random motion of air molecules that impinge on the face of the mic's diaphragm plus the thermal noise that is produced by the output resistance of the mic (equal to the sqrt of 4kTBR, where k is Boltzmann's constant, T is the absolute temperature in degrees Kelvin, B is the Bandwidth of the signal path and R is the output resistance of the mic). We'll call this the microphone noise.
You also have a couple of noise sources right at the input to your amplifier. One can be modeled as a series noise voltage source and the other as a shunt current noise source. The resulting noise at the output of the amplifier is given by:
Vnampout = (Inamp * Rmic * N^2 + Vnmic * N + Vnamp) * A
where:
Vnampout is the noise at the output of the amp
Inamp is the current noise of the noise current source at the input of the amp
Vnmic is the noise voltage of the mic
Rmic is the output resistance of the mic
N is the turns ratio of the transformer
Vnamp is the noise voltage of the voltage noise source at the input to the amp
A is the gain of the amp
The signal voltage at the output of the amp will be
Vampout = Vmic * N * A
So, the S/N is given by:
S/N = Vampout/Vnampout = Vamp/(Inamp*Rmic*N + Vnmic + Vnamp/N)
Typically, the largest contribution to the noise at the output of the amp is Vnmic, but notice how Inamp*Rmic is multiplied by N. You have to be careful not to use too large of a turns ratio, N, or your input current noise of the first amplifier stage after the transformer will begin to dominate the noise budget.
So, if you have mics with lots of self noise, then you can use lots of "free" gain from a transformer with a high turns ratio, but if your mics are pretty low noise, then you have to be careful not to use too much transformer gain or the input noise current of your amp will start to become the dominant noise source. The goal of any preamp should be to get the signal level boosted to a useful voltage level without significantly degrading the S/N ratio.