The reason I write this is because i am tired of people saying that they have to be careful agains explosives when using the DDS
The DDS is one of the items that does not make any sence. The reason for this is that the 20% more explosive damage taken is not a big enough downside. if you take one pill from a demoman without the DDS at full hp, you will end up with 25 hp. However if you take a pill from a demoman with the DDS at full hp, you will end up with 30 hp. It effectivly gives you more survivability against explosives. If you get hit by a direct hit rocket that would deal 125 damage to a SMG sniper, it would also kill a DDS sniper in one shot. Another fun thing about the DDS is that ambassador headshots deal less damage at close range compared to long range shots. The reason for this is that the DDS blocks 15% of the damage from a bodyshot at the same range. And because of this the blocked portion is higher at close range.
Back in the days when this mechanic was a part of the polycount set it was ok because you made a trade. The enemies can't headshot you, however you can't them. This was a fair trade. Now however we all know that a sniper 1vs1 is not decided by who clicks first. It is now decided by who has the low enough moral standards to use the DDS and think it survive with 7 hp then shoot when the other sniper is reloading.
The only trade you make is more survivability agains anything and when a medic fully buffs a DDS sniper it ends up with 225 HP instead of 180 compared to not being able to use jarate or SMG. For most of the snipers in pubs the DDS is clearly a better option unless you like to play relativly offensive as sniper.
Some of the changes to add would be +50% or +60% explosive damage taken. This would lead to you being killed in one shot agains demoman pills and soldier rockets. The option between 50% and 60% comes down to the range a rocket should kill you. This should probably be decided through playtesting.
Another solution that could be added is changing the DDS to what it used to be in the polycont days, people still used it when it had these stats. Then give the "can't get killed by headshots" effect to the Sidney sleeper. Valve has tried to buff the Sidney sleeper a couple of times, and still there are few and far between in pubs. I think this change could make it see more use if that is Valve's goal.
TLDR: The DDS gives you more survivablilty against bullets, fire, all standart meele weapons (- the spy knife, still 4 facestabs to kill), and explosives (read above for more info). Change the DDS to +50% damage taken or give the no headshots mechanic to the Sidney sleeper.