With the -2V of cathode-grid bias on them, the input headroom will be twice that assuming a few idealizations( like perfectly linear valves ). apply a 1V signal ( say a DC voltage ) and the cathode voltage will increase by 0.5V There will be a +/- mu/2 plate voltage change on the anodes( the input section will go - and the grounded section will go + ).

The anode-anode voltage change will be mu*input voltage. Accounting for nonlinearities, most of which cause 2nd order distortion, the actual headroom will be slightly different.
regards,
Douglas