Hey ddafoe,
Without copping out on your question, both the 5.5 and s900 are superlative amplifiers. The 5.5 has a very neutral presentation overall with beautiful timbres/tonality The s900 also has great pristine colors with a smidgen of over all warmth. They are two of my favorite amplifiers. Neither is superior to the other. Based on personal taste and system synergy would lead to which you would pick for your system. Also, both will drive virtually any speakers with ease.
Teajay