I haven’t played a barbarian since early on in D3, so have no idea about how to run one. For Ambo to apply rend, do I need rend active in my skills? I presently don’t and it doesn’t seem to be doing any more damage than when I didn’t have Ambo.
Of you don’t have rend on the skill bar then the rend that ambos applies will be the default with no rune selected, so you’re giving up quite a bit of damage without it.
Could I suggest the build guide, stickied at the top of the forum, which details why you want Rend on your abilities (amongst a whole host of other incredibly detailed and in-depth information about the build)…
You don’t need it, but there’s a rune which increases the damage done directly. there’s also a rune which let’s it spread, effectively giving you area damage. So by not having the skill on the bar you are going to forego more damage from it.
Then there’s the selection of element, which can be really good a lower levels, as the change of rune is not a huge loss of damage, but can make it complement your gear better. For instance my best Mortick’s bracers came with lightning damage, so I went for the lightning rune to get that elemental increase on my Rends, and that rune even comes with a 20% to the skill itself.