Two benefits from adding small subs.
You can high pass the monitors and relieve the woofers of deep bass duty which will clean up the mids and allow your amp to deliver more power without clipping (provided you high pass the signal to the amp).
The second benefit is that using subs allows you to place them in locations that maximize bass performance and response smoothness which might not be where the monitors are best located. A slightly larger monitor will not do for you what a couple of small subs will add.
You can high pass the monitors and relieve the woofers of deep bass duty which will clean up the mids and allow your amp to deliver more power without clipping (provided you high pass the signal to the amp).
The second benefit is that using subs allows you to place them in locations that maximize bass performance and response smoothness which might not be where the monitors are best located. A slightly larger monitor will not do for you what a couple of small subs will add.