A properly integrated sub not only adds heft and depth, it provides a more solid foundation for the sound from your monitors to ride on. The result is definitely more "presence".
REL subs can connect to the amp's speaker connectors, so not having a sub/pre-out is not an issue. New or used a REL would fit your budget. (I generally lean toward sealed cabinet, downward firing subs.)
REL subs can connect to the amp's speaker connectors, so not having a sub/pre-out is not an issue. New or used a REL would fit your budget. (I generally lean toward sealed cabinet, downward firing subs.)