You can read Dale's own words on why he used the tubes for his initial design.
"I started out designing a solid state amp, but quite frankly I got lazy. I thought that Jim's circuit would allow me to determine just what problems I need to address."
Dale had a design from Jim Lefevre which he used as an initial starting point.
He did replace the 6X4 tube with solid state. The 6X4 was Jim's design.
Dale's goal was to use the vacuum tube design, do some testing and then design the solid state version.
You can read Dale's design thoughts in the attached PDF.
Glenn
