Your Theta DAC was a pro audio style device. It output 9 volts on RCA and 18 volts on XLR.
You are experiencing the limitation of consumer grade gear.
Pro audio gear generally has much higher line level signal levels which allow the devices to exceed the performance of consumer grade gear (better signal to noise and better dynamics).
Some manufacturers of consumer DACs use pro audio grade designs (like ARC) but Schiit Gungy does not. It has the same output signal level as standard consumer electronics.
You are experiencing the limitation of consumer grade gear.
Pro audio gear generally has much higher line level signal levels which allow the devices to exceed the performance of consumer grade gear (better signal to noise and better dynamics).
Some manufacturers of consumer DACs use pro audio grade designs (like ARC) but Schiit Gungy does not. It has the same output signal level as standard consumer electronics.