[raytard]

while the factory sub is meant for less power than most aftermarket ones, underdriving a sub never does any wrong (I drive my 1600 watt max 12 inch jbl subs with 75w apiece, just because its almost too loud with that much wattage.)

avalon's of all years require either:
a)(simpler) free air 8in sub (99% of modern subs are box-only, it increases power handling mostly, as well as volume and such, the only one i know of right now is the kicker comp 8inch as far as free airs go, if you find any older (80's to earlier 90's) it is usually free-air and low power handeling)
b) take any 8-inch sub and attatch a box for it, simpleist way is just get a box for an 8 inch sub, and wood screw through the speaker mount, rear deck, and then the box (or is it deck, speaker mount, box, i cant remember if the stock sub is mounted from the top or bottom)

and all avalon's have the cut-out for a sub if you want to add it with a separate amp, but honestly your best bet is to just take the little baffle out and put a more normal sub in the trunk with a normal box.