I have the 80S with a Ref 6 and the Devore O/93's.You definitely do not need the 160S, no matter how large your room might be. Pentode vs. Triode is personal taste. Triode provides a bit more depth to the midrange, Pentode provides more authority, faster and tighter grip over the drivers. I prefer Pentode.
I have the ARC 150SE so I think I know from experience what you would hear-or very close to it- if you were to pair the 160S with the O/96's. You will again hear slightly more authority and even a slight increase in soundstage width and depth but is soooo subtle.
Warning-the 80S takes more time than any other ARC amp I have owned to break-in. Instead of 400 hours I would say it takes twice that before you hear the sweetness and beauty of the amp.
The O/96 is a phenomenal loudspeaker. I prefer it to the O/93. It is less neutral but soooo fun to listen to. In the average room you need no more than 30 watts to get the best out of the O/96. And eight watt SET will drive the O/96's. 150 watts is overkill. Won't hurt, but it won't help much. I suppose if you were into synth or classical with pipe organ and such played at tremendously loud levels that the extra grunt would be useful. Otherwise there is no way that you need that much power.