Any of the Merlin VSMs will provide a heckuva lot more bass extension than your Kef monitors - whether that's enough to satisfy you, who knows?. IME, the VSMs will be -3 db somewhere around 35hz in most rooms of your size. The more recent versions of this speaker tend to be voiced a bit fuller through the mid/upper bass than earlier versions.
Hifiman's suggestion of subs and sats may provide more "thump" down low - which may prove satisfying on rock music, in particular. If you opt for multiple subs and/or EQ, you'll also probably be able to get flatter in-room response below 100hz than you can with the Merlins. But the VSMs certainly have their own charms - great speakers.
good luck
Marty
Hifiman's suggestion of subs and sats may provide more "thump" down low - which may prove satisfying on rock music, in particular. If you opt for multiple subs and/or EQ, you'll also probably be able to get flatter in-room response below 100hz than you can with the Merlins. But the VSMs certainly have their own charms - great speakers.
good luck
Marty