The Viper is a very nice whistle, it has little backpressure, moderate breath requirements, and a pure not-so-airy-as-many-lows sound
The lower notes are strong and the second octave notes are not so loud, it's very well balanced.
The Burke High D is amazing. It's almost perfect. Amazingly well balanced, clean and pure sound, strong lower notes, not-shrill second octave, moderate to low breath requirements, and feels very very fast.
In both Burkes the quality is incredible. Both are very precisely crafted and beautiful whistles. Both whistles are very clog resistant, they won't clog easily and when they do, the sound only gets a bit "different" they won't get muted.
Now about the SYNs, first let me talk about sound and playability: The sound is very close to the Burkes, only more "open" and louder, especially on the second octave. It is less pure, sounds a bit more traditional (but not even close as traditional as a Feadog). The balance between octaves is very nice when compared to my NT WW wich is my loudest whistle at this time.
Playability is great, both whistles feel very fast (not as fast as the Burkes though). They require a bit more breath than the Burkes, but less than my WW. The backpressure is also greater than the Burkes, but way less than the NT WW... so I'll say it's moderate. Regarding clogging, they will clog sometimes (I believe it's because of the small windway) and when they do, they won't make any more sounds. But I used the soapy water method and they didn't clogged again since that.
I really like these whistles, and I agree with all the people that says they're one of the best at this price range.
Now, let's see the other side of the coin: Construction is not so precise, one of the fipples is not well centered with the window, but it's not big deal as it sounds very nice. The problem is the other fipple, it's very out of center, and I feel that it looses some air in the backside. I used a drop of transparent glue and it seems it got fixed, but the windway is out of center too, so I'll have to play it a bit sideways
They also have some small scratches in the finish, but that's easy to fix, just a bit of metal polish and they look terrific.
Overall they are great whistles, but I'm just a bit dissapointed with the one with the outcentered fipple/windway...
