Monoblocks or upgrade to larger amp?

Good question. Obviously there would be a big benefit to adding another amp and running them as monoblocks. Better staging and refinement, but the 250 is an improvement as well. I am not that familiar with the Contours. Do you feel you need much more power? If so, go the 250. If you are very pleased with the sound and just want to better it sonically, I would probably just go with the second 100.

You can always go with 2 SA 250's! :)

there was another discussion recently that applied to this topic where it was thought that bridging the amps didn't produce all that good of sound. Unless you were to biwire/biamp the system, one channel of each amp for the highs and one for the lows or one amp for the highs and one for the lows, bridging would would be required to use the amps as monoblocks. The biwire idea always gets fairly good results though ususally only one amp or a pair of tru mono blocks is involved. More power is always nice for extra bass control and dynamics. I have tube amps with around 230 per side and am wondering what 450 would be like. I'll probably just keep wondering though. For simplicity i think i'd get the bigger amp

Bi-amping will almost always improve sound quality. Actively bi-amping is almost always the best way to go.
Most people will recommend vertical bi-amping, but horizontal biamping will also provide definite advantages.
I personally am actively horizontally bi-amping. The difference from when I just used one amp is mind boggling.
I will never go back!