Hi @noddyspuncture; Rod's amp doesn't use a SMPS, and the particular par starts;
"If you do have a suitable bench supply - This is much easier! Slowly advance the voltage ..."
He's talking about having a variable DC bench power supply, not a Variac, and bringing the DC on the newly built amp up slowly. As you will see just below there he shows a very conventional split rail supply.
To go with my light bulb limiter I have a collection of globes of various wattages ranging from a 10 watt "pygmy" pilot light up to 100 watter, but even testing valve/tube amps I rarely use anything higher than the 40 watt - when you need limiting at all it is generally at a pretty low level.
I also have a couple of blown 3AG fuses with a 150 ohm 10 watt resistor soldered across each, and I normally use these when servicing solid-state amps, particularly rack amps, as they often have exposed fuse clips for each supply rail and I can just plug them in.
The general line of attack with an amp that used a SMPS would be to first isolate the SMPS from the amp and see if it is functioning correctly on its own ('tho it may need a small load to fire it up).
If so, then a power resistor or two can be used between the SMPS and the amp, but if you can't get something reasonable out of the SMPS then you have to start by fixing that first (and SMPS repair is a whole topic on its own).
Variacs are certainly handy, but over a lifetime of servicing I've never really felt the need to get one. Since they are actually a transformer they are still a pretty "stiff" supply and a limiting lamp is a much more effective way of restricting the power input, which is what you really want to do to avoid frying anything.