Man, gotta be honest, if you're trying to compete with a synth with two full bass amp stacks pumping out beats of their own, I'd say you pretty much don't have a prayer of having an unamplified kick being heard. Try all these suggestions if you want, definitely don't go bigger than a 4" hole if you want resonance (you can use a tin can and an exacto knife to cut it)... but at the end of the day, I'd really recommend a mic and a small mixer to get it into the bass amps. They'll take the frequencies without a problem, and you won't have to spend time and money on your kick. And, you'll save your leg, since you said your drumming is pretty busy.
Go buy yourself an Audix D6 or an AKG D112, some small mixer that you can plug the synth and bass drum into, plug it into the bass amps, and go for it. You'll have volume, control, good tone, and a simple setup that doesn't require a PA but still gives you the umph you're looking for. It'll cost you in the neighborhood of $300 or so, but I think you'll be much happier.