well first off, a gain knob isnt a volume control, every amp has a specific setting that the gain should be set at for each specific install. and if your gains are "at full" then you dont have it set up right.
second, subs with that low of power handling are usually not that good of quality, and dont produce as good of sound, more power doesn't necessarily mean louder, quality subs have heavier internal parts and need more power just to be at the same sound level, however they will also have much better control over the movement which translates into better quality, and more accurate sound.
you never want to run any system at 100% (which i think would be the case if you only had 250w) you will have a high amount of heat distortion and no clarity doing so, which will significantly reduce the life span of the equipment, having plenty of headroom with the power will allow the amp to run cooler, and play cleaner, while not having the sub ran at 100% all the time will increase sound quality, reduce heat build up which will increase the lifespan.
but in the end its all up to you and if you think 250w is gonna be enough, then go for it. i definitely would'nt try to run 250W to a sub like a Type R, maybe a Type S, but a R would sound like rubbish with that amount of under power going to it.