Clipping Class-AB Output
AC input is a sine wave, which has a positive swing and a negative swing, and goes through zero twice each cycle. For this zero point, biasing of Q11 seems nearly acceptable.
Your problem arises when Q11's base voltage rises toward its positive peak. Q11's collector current increases: voltage drop across R8 increases. This would cause Q11's collector voltage to become lower.
So you have the situation where Q11 base voltage is rising, and Q11 collector voltage is falling. They meet, and Q11's collector cannot pull any lower.