Mud out of which 2 of the quivers with %Sentry & %CA hit the hardest? I think they are different slots if I'm remembering right except for the quiver.
I go for +sentry, as it increases damage of both cluster arrow AND multishot fired by sentries. I'm starting to think the best way to go is 5 piece marauder + rorg, hexing pants of mr. yau and so on. One interesting build I've seen forgoes any hate builder for spiked trap and chanon bolter. I've been going without any hate builder so that I can use marked for death (contagion). It feels weird, but I've been soloing T6 with it with no problems, just have to watch the hate expenditure and be smart about grabbing health globes.
I use an SoJ, marauder 5 piece + cindercoat, my best dps ammy, reaper bracer, rorg/SoJ, Kuul-aide (Oh yeah!), the best dps 2h crossbow I have (you want one with dex, +10% dmg, socket and as high damage range as possible), bombadier's rucksack. Keep in mind though if you can get the hexing pants, you can go marauder chest, which opens up the possiblity of up to +45 sentry skill (shoulder chest quiver) and +30 cluster arrow skill (helm, boots), with +60 fire on bracer/ammy/rings if you can find top notch gear. Hexing pants give you another +25% damage or so, assuming you keep moving (which is pretty easy with sentry build)
. I don't yet have hexing pants though. :-(
For skills I use sentry (guardian), vault, pet, marked for death (contagion), multishot(arsenal), cluster arrow(loaded for bear), numbing traps, awareness, steady aim, and either archery or perfectionist. Note that the extra 40% damage from ambush is NOT multiplied by the bonus from Harrington's. Strange but true.