For the most part, the only difference between the two is the location of the drivers. With the G25, they are in opposite sides of the cabinet. So the back wave of one driver cancels out the back wave of the other, cutting down on the internal sound waves that can cause cabinet vibrations (distortion). But with proper bracing and a thick (2 1/4") front baffle, cabinet resonances aren't that much of an issue to begin with.
In this case, we discussed the placement options and the G25 would not work without the amp showing from the front of the subwoofer. So changing it to an F25 was a better option.
If you think of it, the best subwoofers in the world pretty much have front-firing drivers. So while the concept behind the G25 is solid in theory, the difference in the performance is minimal. It's one of those audio concepts that are valid in terms of theory, but don't make much difference in terms of what you hear. So go with the G25 if the placement works for you and the F25 if it doesn't. You'll be happy either way.
I hope this helps.
- Jim
