When presented with a balanced input signal source, most tube fanciers' first response is to turn to the classic long-tailed differential amplifier. The reason is not hard to find. The differential amplifier accepts a balanced input signal and delivers a balanced output signal. In addition, it offers an excellent CMRR figure, which means that common-mode noise will be dropped from its output.
This means that the noise at the B+ reflects—in its entirety—to the amplifier's outputs. Another way of looking at it is that the higher the resistance into which the common cathodes terminate, the higher the plate impedance at each output. The impedance at each plate is equal to 2(mu + 1)Rk +rp. Thus if resistance Rk equals 0, the output impedance is simply rp; if resistance Rk equals infinity, the output impedance is equal to infinity. Now a two-resistance voltage divider that holds a bottom resistance equal to infinity does not attenuate much at all.
By the way, if a small valued common-cathode resistor is used, then the CMRR falls apart and the PSRR improves. In other words, forgoing a constant-current source or a large valued common-cathode resistor allows common-mode signals to be amplified almost as much as balanced signals. In addition, effective plate impedance drops to 2(mu + 1)Rk +rp.
Actually, the situation is bit more complex. In terms of even-order harmonics, the distortion on each individual output is greater than the differential sum from both outputs; in terms of odd-order harmonics, the distortion on each individual output matches that of the differential sum. The following graphs are the result of running four simulations in SPICE.