You can perform a frequency response analysis with .bode but not with .ac.
The .ac analysis is for small-signal analysis, which linearizes the circuit at the bias operating point.
However, your circuit is a switching circuit, and its steady state is based on switching between two stages. Traditional techniques in power electronics require a state-space averaging approach to derive the transfer function. However, frequency response analysis techniques utilize a perturbation signal to obtain the frequency response (time-domain simulation + FFT). This can be done in your circuit. However, you have to study how .bode works before you can master this technique.
SOC-D_amplifier_(complete,_BTL)_for_freq_resp(.bode).qsch (55.5 KB)
This is how this technique looks. Well, this is a time-domain technique; running from 1kHz to 100kHz requires about 8 minutes in my desktop. Expect it to take much longer if you run from 20Hz.
Reference about .bode or FRA approach
.bode of Qspice
Frequency Response Analysis (.bode) Study Guide - QSPICE - Qorvo Tech Forum
FRA is module that written by community to perform analysis likes .bode in Qspice.
FRA by @physicboy
QSPICE/FRA_project at main · physicboy/QSPICE
@physicboy Do you have a forum post link to your FRA-V7?
FRA by @KSKelvin
“A Frequency Response Analyzer (FRA) in QSPICE” - QSPICE - Qorvo Tech Forum