I have Definition 2, Druid Mk 4.08 and Credenza in my systems and have heard Essence in my house for direct comparison.
Essence is in many respects a "Super Druid," with extended bass, better horizontal dispersion and a more linear, extended top end than Druid. Retaining the simple single FRD + supertweeter architecture of Druid, Essence retains the essential tone and directness of the earlier speaker.
But Essence is not, unsurprisingly, in the same league as the Definition 2. The elastic sonic scale possible with the dual FRD configuration and 4 x 1-" sub-bass array, plus the acoustic management of high frequencies (along with mitigation of floor and ceiling effects) rendered by the FRD-ST-FRD drivers placement, yield resolution soundstaging just not available from Essence.
This is not in any way denigrating of what Essence is. Definition 2 and Essence are two different loudspeakers and very different price points, Definition 2 costing about 3X Essence. Essence plus a subwoofer will not equal Definition 2s or even Definition 1.9 if any are left, no matter how good the subwoofer.
I'll further say that you don't need a subwoofer with Essence anyway. It's flat to about 30 Hz (and sounds it), with useful signal deeper still. Most people don't have a room to support deeper bass than that, and the bass quality from Essence is better than 99% of subwoofers. Essence is a striking bargain, which allows you to keep things simple. Better to sell your sub, and put the money into better amplification. With Zu speakers in a system, the fulcrum of fidelity moves to power amplification. It's the power amps that will further define the character of your system.
But don't expect that you will have an equivalent to Definition 2. That is another level of investment entirely.