The cop was giving you a break by only writing down 1 12" sub - the fine is $25/sub so your $50 ticket is now $25.
One of the perpetuating myths of ticketdom is the "there'* a mistake on the ticket, so it'* invalid" fable. Sure, there'* going to be the rare occasion that a ticket gets tossed for a recording error, but that'* not the norm. Cops & judges have heard pretty much every excuse made, so this isn't new territory for 'em. Not worth the hassle to fight a 25 bucker.