The resistors Rb serve only one purpose: They must allow a bias current Ib between the base nodes and ground.
At the same time they are a part of the dynamic signal input resistance. Therefore they should be as large as possible, however, low enough not to cause a dc voltage drop that is too large.
That means: The value should be a trade-off between two conflicting requirements: A typical situation for analog electronics.
Hence. some values between 10k and 100k seem to be appropriate.
Example: For a collector current of 1mA and a current gain of the BJT`s of B=100 we have Ib=10µA. Thus, a value of Rb=10k would produve a dc voltage of 0.1V at the base node. This seems to be acceptable.