How does one calculate the input impedance of a tube based preamp?
The input impedance is almost entirely set by R2/R1, so it's about 47K || 1M or about 47K. 

You can increase R2, for example to 5M and leave out R1. 

A tube behaves much like a JFET (which, in turn, is like a depletion-mode MOSFET with a diode from gate to source).