Point-to-point wiring throughout.
Point-to-point wiring is the term given to a style of construction where the components are mounted on the tube sockets and/or terminal strips, and the connections between components are then hand-wired together to complete the circuit. As compared to printed circuit boards, there is less capacitance between conductors in a hand-wired amp, since most of the connections are separated by air instead of the fiberglass of the PCB.
There are actually places where this matters: For example, tube grid circuits which should be wired for minimal capacitance (few surrounding conductors), as this capacitance can roll off the high end. Though more time and labor intensive than the use of printed circuit boards, point-to-point is more robust and superior for the high thermal output commonly associated with tube amplifiers — and is virtually trouble-free!