Regarding tube preamps would an output trandformer in place of the capacitor eliminate or reduce this potential fluctuating impedance mismatch ?
The issue with an output transformer is loading. If loaded at an impedance too low, it will roll the highs. In addition, the tube driving the transformer will see a lower impedance and so will make more distortion.
If the load is too high the transformer can ring. This will add brightness. In some cases where the loading is really way too high, the transformer will begin to express the inter-winding capacitance as part of its output (rather than the turns ratio), causing a loss at the frequency extremes and what response there is will not be flat.
Amplifier input impedances can vary by 10:1 so it is helpful to know what the transformer is set up for. The manufacturer of the preamp can supply that information.