Hi there, I see there are some mixed impressions of the Soundwriter so I will add my 2cts. I have been listening to a custom version of the Soundwriter since Friday, and have the A18.
The Soundwriter has a neutral tone with an energetic presentation following its upper mid/lower treble lift. Its bass is different than the A18; the emphasis is on sub- over mid-bass, and it has less upper-bass than A18, especially with the M20 module. The mid-bass is quantity is still north of neutral, but its decay is very quick. Therefore, the mid-bass is present but contained, so mid-bass lines do not warm the stage or the midrange, resulting in a transparent midrange with a neutral tone. So personally, I would not label the Soundwriter as warm overall.
For reference, I would also not classify the A18 as warm, even with M20 module. The A18 has signficantly enhanced bass, but compensates with peaks in the lower and upper treble. While the Soundwriter has less upper-bass, it also has smoother and less prominent upper treble, which benefits its coherency; it places emphasis on the main components of a track (specfically vocals, guitars, and drums) without overarticulating finer detail. Which isn't to say the Soundwriter isn't smoother than the A18 overall, due to its upper-mid tuning.
A simple way of viewing the Soundwriter is an A18 with carrot module. However, with the carrot module the A18 sounds closer to the M20 in terms of bass and tone (a bit bassier even due to the closure of the ports), where the Soundwriter is tonally closer to the M15. However, as mentioned there are differences throughout their signature. The Soundwriter has a more bodied and transparent midrange and different bass/treble tuning, which results in a different tone. Conversely, the A18 for instance has a wider stage, smoother upper mids, but relatively more prominent upper treble.
As a sidenote, I also heard a demo version at CJ NY which I didn't like as much. That version I found a bit bassy, and the upper treble more prominent. However, I have heard MS brought the latest demo version closer to the custom version, although I haven't heard it myself.