What's your limiting factor? Money? Storage space? Lots of guys run Memphis full range 8" in their bags... another option is 6" subs ... another option is the excellon 10" kit by American Hard Bag... you have choices
Check out Harley Davidson Bagger Stereo on FB.... lots of great info and plenty of great folks to help you out...
I have lids... 600 watts and I love my system... I have a lot more bass, but not as much as you would think from lids with 5x7's... they are not as dynamic as one would think... but remember this... once you are moving... the sound from the lids become non-existent...i would like to keep my bags open but i am open to speaker lids but ive heard different opinions on them, people say the driver cant hear them once up to cruising speed.