My guess is solid state loop, 4 gain stage design with the tone stack being plate fed. I've built a few amps that way and you'd be pressed to hear a difference between plate driven and cathode follower driven. However, the original 5150 had 5 gain stages with the tone stack being fed by an inverting feedback amplifier, so 6 stages total) and there's no way you can do that with only 3 12ax7 with one most likely being the typical long tail pair phase inverter.