That's a strange calculation.
If you get a passive sub, you are done.
If it is active, and really want it passive (why on earth) the parts will cost you $20 max, and can sell the 2EQ preamp easily for $50...
It's your money, do what you want, but it is sounding like a false economy now...
Like I said, I've yet to find a passive SUB for sale anywhere.
I don't want a preamp. Just volume and tone. No batteries.