Well since SO many people would like to know, yet no one knows for sure, I decided to break down, take the time and money to test this.
Ensign was mostly right in his post about this.
I tested first with a full set of ringmail (AL 50) and 2 pieces of ascalon armor (with reduces dmg from attacks). Both have the same AL (verse magic), so things could be consistent.
To be straightforward, ascalon pieces do NOT stack with one another. IE. you get the full effect from just one piece. Additionally, I had not know this before, so I did not get a result for the next question, but absorbion runes ONLY absorb PHYSICAL damage! I tested this with 2 pairs of regular ringmail with one having an absorbtion rune. I got hit by several ice golem water tridents, which have a consistent damage (unlike physical). The damage was the same throughout.
I then took an average of physical attacks with the 2 different pieces from several types of mobs (ice golems, sand drakes), the minor absorbtion rune reduces PHYSICAL damage by 2, equivalent to ascalon armor. I assume major abs = -3 and superior -4 but did not test, as I don't want to buy another chest plate
I also tested the absorbtion rune versus aftershock (sand drakes) and found the damage to be consistent with and without.
I equipped a full set of droknar's plate (85 +10 physical) except for the chest (most often hit piece), which was AL 50 (+20 vs physical). I had a second AL 50 chest piece with an absorbtion rune in it. I MADE EXTRA sure that aftershock hit the chest piece because the damage was almost double that of any other piece (59 on any piece except armor but 125 on chest). I then switched to the chest piece with the abs rune and recieved 125 when it hit the chest (where it should have been 123 like it was with an ascalon piece).
So, the only thing I am left wondering is if ascalon stacks with absorb runes in the sense that one reduces damage from all attacks (ascalon or knight) and one reduces only physical. If this is the case, the most suitable armor at level 20 is 1 piece of knight for dmg reduction verse magical (-2) and a full set of plate with a superior abs rune for -4 physical.
A quick summary because this is a lot to take in and probably confusing:
*Ascalon and knights pieces do NOT stack with one another. If you MUST have this armor, get only ONE piece!
*Absorbtion runes only absorb physical damage (as seen versus water trident and aftershock). The only thing I can think of why this might not be the case is if those attacks ignore absorbtion, which is doubtful as ascalon and knights reduces the damage by 2 whereas abs runes do not.
*Minor absorbtion runes reduce physical damage taken by 2.
I might test knights+abs rune later tonight. If anyone would like to verify these results, please do and GL.
Edit: Also not sure if knight absorbs more than ascalon, will test that too.