SINE WAVE
lots of free sine wave generators on the internet, just goggle it and have your sine wave generated in a few seconds, in .mp3 or other compression and encoding formats of your choice.
Then simply play it repeatedly using an mp3 player, or a PC, a tablet, connected to the input of your bass amp (mine has a "line" input along with one dedicated to the bass)
SIMPLE
It's much less simple to understand how much watts you want to put into the cab using that 25 hz sine, because the tone is so low that our hearing isn't so sensible at that frequency. Just use common sense and do not smash your new speaker searching for its excursion limits with a powerful amp.
I think this is the reason because mr. Claber does not suggest using this method anymore. The alternative is to just play the damn thing at high volumes, it will take more time, but it's equally effective