OK sooooo i want to know is this build viable, i mean ive been roflstomping people with it lately and its kind of awesome.. but im just wondering about other peoples oppinions.
So here is the overall super late game goal build, take in mind i go philo into HOG with cloth and 5 pot as my starting items but late game i want prefferably. (no particular order)

Boots of swiftness
Rylais scepter
Rabadons
Sunfire cape
ROA
Will of the Ancients

Note: This is the build for a team with a small ap comp, if they go double ap or an ap jungle i forgo rylais for an mr item usually FON
9-21-0 masteries
AP per level quints/glyphs (only appropriate runes)
magic pen marks
armour seals

End Result: You have over 4k hp and about 150 armour so you have 60% damage reduction. Also you have about 620 AP making your ultimate do over 900 damage to their entire team. And with your constant aoe from your tears, tantrum and sunfire cape you are constantly dealing big damage in fights, slowing people with your rylais proc and healing yourself with spell vamp. And your WOA helps out your team as well.

Oh and random qs, does sunfire capes passive proc spell vamp? it does do magic damage...
anyway last games that are still in my match history with this build are:
6-0-8
15-1-10
7-4-12

NOTE: NEVER GO THIS BUILD FROM BEHIND! If you have a poor early/mid game, build philo>HOG>aegis>WOA maybe depending on team comp and just be reactive from there countering the other team

If your team lacks a mage; i can understand going AP, but if your team is lacking a tank: go tank.

Your mission as Tank amumu is to survive fights, stun's, apply his -m.res debuff and do damage mitigation (his cry W ability). The only Ap item's you would ever build with tank amumu is abyssal scepter.

tell that to dan dinh

i would suggest rod of ages abyssel scepter hour glass and lich bane. hour glass for place well played ults due to his low life. the rest is filler based on what the enemy team is and has, of course magic pen boots

"OK sooooo i want to know is this build viable, i mean ive been roflstomping people with it lately and its kind of awesome."

If it works, it works. Just be aware that your team likely expects Amumu to be a tanky stunbot with some AoE damage on the side - consider letting them know ahead of time in Champ Select, or be willing to switch gears mid-game.