The is my new project in Oct 2009. Making use of my original F4 casing and power supply, I can complete the two sets in about a week. One set is using IRFP240/9240 mosfets and another set is using Toshiba 2SK1529/2SJ200 mosfets. I only adjust the bias to about 1.2A which give about 20W at 8 ohm load. Also I do not get the super low distortion that Nelson Pass got in his measurement. According to Nelson Pass, the distortion can be trimmed using a source resistor at the input jfets but I am too lazy to do that as I am very happy with the sound!
The sonic signature is very much similar to F4 but with a bit better control on bass. I do not hear much difference between IRF and Toshiba F5 but the distortion of Toshiba set is almost double of that of the IRF set.
Here are some pictures of the IRF F5 and some measurements that I have.
F5 IRF distortion vs input level