If the 8 ohm resistors are 20W, then two 8's in series will give you 16 ohms at 40W.

If you need 25 ohms, use three 8 ohm resistors in series for a total dissipation of 60W at 24 (close enough to 25) ohms. Power resistors are like what, + or - 10% ? So three 8 ohm resistors will give you something like 21.6 to 26.4 ohms, depending on how the tolerances stack up?

What's going to get kind of silly is making an 8 ohm, 100 watt part. You're going to have to series/parallel so may individual pieces it just won't be practical.