Am using a passive preamp (home built with goldpoint 25k resistors). Amp has they typical 100K input impedence.
This isn't all you need to consider. If your 300B amp has something like a 12AX7 sitting at the input, the miller capacitance is going to tax the high frequency response of your system when you factor in the impedance of the passive driving it, and adding in the cable capacitance.
If your amp has a pentode (wired as a pentode) at the first stage, this issue will be greatly decreased.
You could also consider changing the 100K resistors at the input of your amp for 250K resistors, but this isn't going to help if the input capacitance is too high.
-PB