I bought four $30 tickets in the first row of 318 for the Saturday game on the Thursday before the game so they were available at that point. I suspect that they will open up the 400 level for the playoff games. [...]
Possibly, possibly not...here are United's home attendances in the playoffs since winning their last MLS Cup:
30th October 2005 (first round vs Chicago) - 20,069
29th October 2006 (first round vs New York) - 21,455
5th November 2006 (conference final vs New England) - 19,552
1st November 2007 (first round vs Chicago) - 19,438
...about what United were drawing in regular season back then (and normally accommodating in the lower bowl), maybe a pinch more.