questions about augment crystals and shavarath sets

[Acaios]

i`ve recently reached lvl 20, and was wondering how augment crystals work on epic items.
i read the ddowiki page : http://ddowiki.com/page/Epic_Crafting
but it was just the other day in the marketplace that someone was selling via trade channel, a crystal like that, that i saw that it had a duration of 90 hours, if i`m not mistaken.
so, is this how it works? it`s just like the guild slots? with a duration. coz ddowiki says nothing about it.

if so, is it worth doing so? i havent run any epic quest yet, but it seems quite some farming to get those 30 epic tokens to make a crystal, just to vanish after 90 hours of gameplay. 30 epic tokens will drop in about 30-40 epic quests, which will take about 40 hours i guess, which is half of the duration of the crystals. (they pause when you`re in public areas, right? but doesnt stop while waiting in explorer`s areas, uh?)


and the second part about the shavarath sets

http://ddowiki.com/page/Unique_items_by_set_bonuses

Assassin

Awanahu Sash - Belt: Constitution +6, Proof Against Poison [Sins of Attrition]
Whisper Ring - Ring: Dexterity +6, Exceptional Intelligence +1, Incredible Potential
Set Bonus: Bonus to hit and damage with sneak attacks (+2 to hit and +5 damage from attacks considered sneak attacks[both nonstacking even if it say exceptional]), 20% less threat
If have Assassin III Enhancement: chance with weapons to apply a Level Drain effect.

Ninja Spy

Nyoko's Necklace - Necklace: Invisibility (CL:15, 3/3), Constitution +6 [Genesis Point]
Nyoko's Band - Ring: Wisdom +6, Exceptional Dexterity +1, Incredible Potential
Set Bonus: bonus to hit (+2 exceptional bonus to hit) and damage with sneak attacks (+5 damage from attacks considered sneak attacks[nonstacking]), 20% less threat



Shintao Monk

Shintao Cord - Necklace: Constitution +6, Concentration +15 [A New Invasion]
Kyosho's Ring - Ring: Wisdom +6, Exceptional Strength +1, Incredible Potential
Set Bonus: +2 Exceptional Bonus to To-Hit and Damage.
If you have the Shintao Monk 3 Enhancement, your unarmed attacks bypass Good and Evil damage reduction.


Tharne's Wrath

* Tharne's Bracers - Bracers: Dexterity +6 Dexterity +6: This item makes the wearer more nimble and dexterous, granting a 6 enhancement bonus to Dexterity., Armor Bonus +6
* Tharne's Goggles - Goggles: Backstabbing +5, True Seeing, Search +15Search +15: Provides a +15 competence bonus to the wearer's Search skill., Spot +15Spot +15: Provides a +15 competence bonus to the wearer's Spot skill.
o A Vision of Destruction Loot (Suulomades warded chest)
* Dragontouched Armor - Tharne's Wrath Sovereign Tier Property
o 2 Item Set Bonus - Ghost Touch and -20% Melee Threat
o 3 Item Set Bonus - Feather Falling



the questions are:
1) in the assassin`s and ninja spy`s set, the to-hit and damage bonus are said that dont stack. they dont stack to what? to each other (meaning having both these sets on)? or also they dont stack to others sets also, like the shintao monk`s that gives +2 exceptional to-hit?

2) do they stack with +5 sneak attack from tharne`s googles?

3) does the -20% threat stack with the -20% theat from the tharne`s set? if it does, is it like healing amplification (which is calculated by multiplying the bonus - <as long as they are different numbers> http://ddowiki.com/page/Healing_Amplification ) or a cumulative 40%? or they dont stack at all, either because a)they are the same number so it doesnt stack, just like healing amplification or b) because only higher number counts - which is the same anyway.

thanks in advance for any input.



EDIT:
and two last questions i just thought of.

1)what`s the AC of epic mobs and bosses? so that i know and plan of how much my to-hit should be..
2)do you put on your melee toons, who can cast spells and/or use scrolls (e.g. tempest), madstone boots all the time (for the hidden effect) or just use it as a clikie at end fights?

[Waylayer]

The 90 hour augment crystal is a lootable item from Devil Assault (I believe). It doesn't require any Epic Tokens to create; and is tradeable.

Regular augment crystals are purchased, with epic tokens, from the NPC behind "The Twelve"; are bound and do not expire.

[Acaios]

that`s nice!

are bound and do not expire.

does that make BTA items (epic stuff from crystal cove) BTC?