The new shoreline The managers at Shoreline Amphitheatre say the improvements they made this winter to the 22,000-seat outdoor theater's sound and video systems were like upgrading a home entertainment center from simple stereo to massive surround sound. Judging from last week's opening night, that's a pretty apt analogy. Shoreline's building manager, Matt Prieshoff, beamed Tuesday as the venue's 20th season opened with a Jimmy Buffett show that brought in 15,500 people, even though most of Buffett's party-hearty fans were probably not thinking about the 38 new speakers on the lawn, the two improved daylight screens above the stage, and the large video screens over the bathrooms. And they probably didn't notice the raised boxes for customers in wheelchairs. ``We're in the back, but it still sounds great,'' said Hilary Faust, 21, a Stanford University student who said she plans to return for more shows. That will please the theater's mana...