I have some Paradigm Reference Studio 40s that have very good bass for their compact size. I am sure there are other speakers that are better, but I can recomend these from personal experience.
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It all depends on how much larger an enclosure you can fit there, and how much sensitivity you're willing to sacrifice.
Just keep in mind that it doesn't take much of a loss in sensitivity to require the amp to reach its peak output on transients (clipping). It might make more sense to add a small powered sub or two that can also fit on the shelf and keep the bookshelf speakers that have served you well up until now.
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.