There is Still a chance that the SO6P had his Powers before absorbing the Juubi, That is possible if TWO conditions are true.

1) If the Sage had always had a Super strong life force and Increased spiritual energy....
2) If The Older Son's Eyes were the Sages original Eyes, And Mokuton and the younger son's life energy was the Sages Original State.

The reasons to suspect this are, A) The Older Sons eyes were not Sharingans, they were some kind of spiral design, granted this could have been an EMS, but we don't know that for sure B) I can't say that the Juubi has used Mokuton while not being under direct control of Obito or Madara C) Madara obtained the power of the SO6P without the Juubi, so the SO6P himself could have had similar powers before absorbing the Juubi.

Until I know more, the above Hypthesis makes more sense to me than any other. It is far more credible that someone who had some serious powers would have been able to stand a chance of subjugating the Juubi (in its final form at that), than a guy who just had Uzumaki life force and knowledge of seals

Anther interesting Fact:

In the Panel where the SO6P was talking to the Baby Bijuus, He, the SO6p had removed the Juubi's Chakra from himself and had used it to create the Bijuus. This is a fact. It is true however that SO6p could have retained a portion of the Juubi's chakra in himself at the time.

The reason for doubting the other explanation is that The Juubi is so crazy strong, and it knows how to defend itself , for example it created mini juubis to attack, and it split off part of its body being burnt by amaterasu.

Last tHought:

Who knows, maybe the SO6P and The Juubi were two sides of a coin. One who had powers similar but weaker than the other. One a Yin, the other a Yang. Maybe the powers that Gave the world a Juubi also gave it a Counter Juubi...SO6P.