An independent Sirius XM Holdings had an encouraging debut Tuesday, as Berkshire Hathaway emerged as the largest shareholder with an estimated 25% stake—replacing Liberty Media and its control holder, media mogul John Malone. Sirius XM Holdings stock gained 2.6% Tuesday to $27.38, after trading as low as $24.43 earlier in the session. A combination occurred late Monday of Sirius XM with Liberty Sirius XM Holdings, a tracking stock that held about 83% of Sirius XM shares.