It's not you, do the math.
Suppose you run the amp 60 hours a week, and on average, it draws 500W (which is probably twice the correct number). In one month, that's 120 kWh. Multiply that by your rate, say it's a whopping $0.10/kWh, and you're paying $12. Use the real numbers and you'll see it's not your amp.
She's not going to buy it, though.
I just moved into my girlfriend's place... she's been here for over a year and just sent this email.
Hey - I just got my Xcel bill and it's $XXX - that's the most it's EVER
been in that apartment even in the dead of summer running the a/c all
day. Are we using a bunch of energy on something?
Oops!