Compatability of ASL HB-1 & Grado RS-1 'Phones

I own an ANTIQUE SOUND LAB HB-1 headphone amp and am going to buy a pair of GRADO RS-1 headphones, which have a 32 ohm impedence. Does anyone know how the HB-1 will power these 'phones? It's max output into 32 ohms is 15 mW. I don't know what this means, however. I like my music loud and wonder if the HB-1 has enough gain. Please get back to me if you know. Thanks for your time and consideration.