I've been playing the head for about a year, and Alan (Chef) and I helped Andy with the manual back in 2020. While I respect the excellent musicianship in the video I will admit that's not my usual playing style or tone goal. However, the feature set is well described and I suspect that more than a few people will be impressed with the level of player finesse/articulation and tonal versatility this head can accurately deliver.
I have used the D-350 with my Berg CN212 and SW 112/115 cabs on country and rock roadhouse/VFW/American Legion kind of gigs (60-100 people). I've also used it in small to medium venues with the SW 112 or SW 115 and a couple times even with an Eden EX110 for more acoustic/folks/bluegrass kind of things. It does the full low end, smooth, clean thing really well...and that's more my style.
During my initial playing I was reminded of a Bass Gear Magazine review Alan did years ago when the Carbine M3 was introduced (after he had already played the M6). I don't recall his exact words, but it was something to the effect that he thought the M3 was more like an M5 as it just didn't seem that less powerful than the M6. The D-350 struck me much the same way in comparison to my D-800 (which I admittedly never use to its full potential). Despite its size, the D-350 can get some serious volume especially when running at 4 Ohms with dual driver and/or dual cabs. IME on lengthy rock and roll sets it never even got warm to the touch.
I recall discussions with Andy and Alan during Andy's concept development and design work. First, this is a professional quality amp with features and capabilities that many Mesa customers had been asking for - including smaller size and passive (i.e. silent) thermal management with a very low noise floor (think studio work). It may come as a huge surprise to many of us, but not everyone feels they need a million Watts at 2 Ohms along with more EQ and signal routing features than a state of the art recording studio. This head was specifically designed to fill a space in the SW line that was not fully addressed by the D-800 for those customers.
The only real trade-offs you have in this head IMO are lower power output (which has yet to surface as a problem in the gigs I've played) and no 2 Ohm operation or deep switch for some obvious reasons. To get the small size, silent running (no cooling fan), and preserve headroom the amp is restricted to 4 Ohm and above loads...once again, not a problem for how most people who would buy this head will use it.
The bright switch (which Andy rightfully credits Alan with convincing him to include) is excellent. I hate bright switches in general, but this one is musical/seamless and just opens up the top end in a way that allows nice articulation without hiss or other annoying artifacts. If Reggae and Dub at volumes where you set off car alarms are your thing, this probably isn't your best choice in the SW line. That said it is not bass shy and while naturally warm feels a little less dark in its inherent voicing than the D-800. The bright switch subtly brings out the upper register and can get you a touch of sparkle in your tone with added treble.
Between the price point, quality (it's a Mesa profesional head with a 5 year warranty), and set of features many players have been asking for, I think this head will be very appealing to many folks who found the current SW heads to be more than they needed or not quite what they wanted. I'm always good with smaller and simpler if it can do the job, and the D-350 is more capable than its size and power rating might suggest. Even if it's not right for you, I think you'll come away impressed with the design and have a fun time trying it out.