Yes, you can see the tiny hole of the mic on the back. The Ultra is just a pre-amp/streamer, no amplifiers, no speakers. It doesn't know beforehand if the connected chain introduces any delay. It does introduce its own delay due to digital EQ. Since all WiiM streamers/amps are designed for multi-room systems each of them has to know the cumulated delay of all components down to the actual speakers, so audio can be played in sync in all rooms.
That's what the mic was for in the first place. Many subwoofers (mostly those using using their own DSP) also introduce a latency which might be very different from the main speakers. So WiiM added automatic correction of the time delay between mains and sub.
Each ms of latency equals a difference in physical location of ~34 cm. So 12 ms difference in timing is roughly the same as moving a speaker by 4.10 m. This might or might not be distracting in a multi-room setup, but there's more to it.
What everyone had been hoping for when this feature was announced is something different. In a certain range around the crossover frequency both, the sub and the mains, contribute to the total sound pressure level. If the signal from the sub and mains are not in phase (arrive at the same time) they will partly cancel out each other instead of adding up, as expected.
4.1 m equal the wavelength of a 84 Hz sine wave. At half the wavelength, full cancellation would occur (at least in theory, in practice the reflection will smooth out the effect a bit). We had been hoping for a function that would automatically delay the subwoofer or (much more common) the mains so much that their combined signal would add up perfectly in the listening position. But that requires the measurement to be taken right in the MLP, e.g. using the phone's mic. Didn't happen.
The physical dimensions of the Ultra are so small it probably doesn't make much of a difference, but yes, if you really wanted to follow this route you would have to place the Ultra's back in your MLP.
That's one way to do it. Or simply try out different delay settings and stick with the one that gives the highest SPL at the crossover frequency.
While you're at it doing some REW measurements you will probably notice that reality is even more complex. The "acoustic center" of the sub is usually not right where the cone is, but further back. Sound reflected from the walls adds to the direct sound and causes this behaviour. For low frequencies there's a similar effect for the mains, of course, but they are usually further away from room boundaries. The result is that the delay you need for a maximally flat summed frequency response might be different from what you would measure with a tape ruler.
Room correction EQ can partly compensate for that (in a sweet spot) but starting with a proper delay can widen the area where the summation works well. If you have the tools and just know about how to use them, I would say: Go for it.
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