It should easily make 50mW into 600 ohms. By my calculation, that's 105dB in each ear. For comparison, THX movie theaters are rated for 105dB with both channels operating.
That should be adequate, unless you have some serious hearing loss - or want to acquire it! :^)
The SEX amp at the 32 ohm setting would give 100mW (108dB each ear). With the Smack transformer, the SEX could make 400mW into a 600 ohm load at the 128 ohm setting - that's not a standard offering, you'd have to consult Doc B about the substitution. It's not quite as refined a sound as the Smack, and of course you can't drive speakers unless they are 16 ohms or higher - but it is the high power option.