I've taken a small, inexpensive practice amp and done an easy, easily reversible, non-invasive, mod that allows me to use the combo as a head with various different cabs.
It can make a mediocre sounding little amp sound a lot better, and "bigger". Depending on how bad/small the original speaker was and how good the cab is the difference can be quite amazing. The speaker swappers are on to something. Much of any amp's tone is in it's speakers.