1V output for 126mV input is 18dB (not 15dB)

If your DAC produces 2V nominal, then preamp output will be 2V x 1/0.126 = 15.9V - way too much for an amp with 2V nominal input.

Perhaps your DAC has jumpers to reduce gain by 10dB (like in my Benchmark DAC), otherwise you can insert 10dB attenuator. Best place would be between amp and preamp, assuming that your preamp can output 15.9Vrms - if not, then between DAC and preamp.

If your DAC produces 2V nominal, then preamp output will be 2V x 1/0.126 = 15.9V - way too much for an amp with 2V nominal input.

Perhaps your DAC has jumpers to reduce gain by 10dB (like in my Benchmark DAC), otherwise you can insert 10dB attenuator. Best place would be between amp and preamp, assuming that your preamp can output 15.9Vrms - if not, then between DAC and preamp.