Matt Erickson (KK5DR) recommends an inductance value between 30 and 300 microHenries to be sufficiently robust to cover 160 thru 10M. His writeup on "Building your own tube-type amplifier" is well done and has some excellent construction information and techniques.
The ferrite mix does not appear to be a critical factor.
Visit his web site http://kk5dr.com/ampbuilders.htm
and send him an e-mail. Matt is a great guy and is very willing to share his knowledge of amp construction.
On his suggestion, I changed my plate parasitic choke to a more stable design. No longer is it the old, typical wire wrapped around the 100 ohm resistor. And it is much more stable on 10M.