I've come from a place of being mildly curious about low whistle and therefore am not terribly likely to be buying any more to keep investigating or comparing. Never say never, though.
The first one i bought years ago was the Howard. I like the big rich creamy low octave, but i find the high octave A & B basically impossible to blow. It is pretty old, and i know there have been lots of changes to these whistles, so i can't say how it compares to the Howard whistles of today. It sat on a shelf for years and years and that was that. Someday i'd try again.
A couple of years ago i was at a friend's place and tried his Kerry Optima. What a difference to the Howard, the high notes were SO much easier to reach. The tone is not nearly as nice, but it played so much better for me that i bought one, and now at least i had a low D that i could play.
When my dad died in 2020 and i had some inheritance $$$ to play with, i thought ok here's my chance to try the MK that i keep hearing about. I admire it as a beautifully made object, but as an instrument i just plain dislike it. It's nice and loud but to me it sounds so veiled and windy, and again the high A and B are really hard to blow. I get that some people do like this sound, but it's not what i want out of a whistle at all.
I ordered the Humphrey on a whim, i was ordering a high D and i thought well, let's add a low D and see how it compares to the Kerry Optima. I was blown away. It sounds big and full and (relatively) pure, and the high notes play the easiest of all so far. Again, to me it kind of plays the most like a high whistle, only bigger. The holes are the biggest (maybe some not as big as the Howard) and the stretch very slightly the longest.
Finding a "low whistle that plays like a high whistle" was never my quest, i was never really on a quest to begin with other than "at some point to find a low D i can get on with". The Howard didn't tick that box, but i didn't bother looking further for i-dont-know-how-many years. Honestly i'm not a capital-L Low Whistle player, i'm just a regular whistle player, and in my case it turned out that i'm maybe too stuck on the high whistle, and i'm maybe just plain too lazy to put any extra work into playing a whistle, any whistle. So maybe the Humphrey is perfect for the lazy whistle player, and i'm fine with that.
Penny