| Well, your terminology is so far off that it's hard to tell what you're really going for. An amp head doesn't have a load, but it NEEDS a load. So you can't change the head to "...make an 8 ohm load". If you're talking the B-25 cabinet, then the only way to change its load is to change the individual drivers (speakers). There are people who have espoused the idea of using a dummy resistor load to change the impedance the amp sees by plugging a dummy load in with the speaker cabinet to get an optimized impedance. But, it does absolutely NOTHING for your sound. If the amp puts out more power at 4Ω than at 8Ω and you use a dummy load to change the impedance the amp sees to 4Ω, the extra power is consumed by the dummy load doing nothing. So, you have more power but no more acoustic volume, and you're generating a lot of heat at the dummy load.
It could be that what they're referring to is to put a 1/4 PLUG (the jack is the female part, the plug is the male part) into the external speaker jack which MIGHT switch the taps on the output transformer so it will work to deliver maximum power into an 8Ω load. IFF (that means "if and only if") the output jacks are switching jacks AND the extension jack switches the transformer outs the correct way, it might do what you want. But I wouldn't go plugging stuff into a tube head based on some vaguely worded and highly suspect conjecture.
Get a schematic of the amp and see what's going on at the output transformer.
John
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