The used opamps are +/- 18V max.
The input voltage = 48V and this is +/-24V.
There is at least 6V available for regulation but maybe even more.
The reason for the rather low dynamic headroom is the method used.
Passive RIAA correction, which some believe is the 'best sounding' method.
With active RIAA the gain varies with frequency. Passive ones have a huge gain (full BW).
This basically means the highs have the largest amplitude (that's why RIAA is needed) and thus these clip first.
Below the basics for this type of RIAA.
When the overload character of the used opamps is 'nice' ticks that far exceed the max. input level are actually less audible (as they are limited in amplitude by the clipping) than when there would be a huge headroom.
The build costs of this amp are below $ 200.- when built in small quantities so that's quite a markup.