You could always try a Bill Lawrence and an EMG H4. EMG says it's the passive version of the 81, so you shouldn't have any impedence issues since they're both passive. Even better, they can be found cheaply if you look.
n/m, I just read your post completely. The EMG HZ that comes with your ESP is the H4 model. If you don't like the sound of it, then either it doesn't sound like the 81 in passive form as EMG says, or maybe the 81's not what you're after.
GBT