Originally Posted by ADRONICUS

What is stated about the PCV valve "could" be true but it is on the lower end of possibilities. To rule this out inspect the air filter and see if there is any burned out looking spots on it. That is a good sign something is wrong with the PCV valve.

Moving on. Backfiring is commonly caused by un-burnt fuel igniting from the heat of the exhaust, thus causing combustion and the backfire. With straight through exhaust it will be more noticeable as well.

1. You already replaced the cat so rule that out. Did you notice a reduction in the backfiring after doing this?
2. If the o2 sensors were bad you would throw a CEL.
3. It could still be running rich which is calling for an adjustment to the fuel supply and a tune on the timing.
4. A resonator and chambered muffler may eliminate this as well. I am saying that because when I had my greddy exahaust it was straight pipe from the cat back into a straight through muffler and it backfired like a bitch. When I switched to the Apexi WS2 it all went away.
5. Have you noticed if the exhaust tip is heavily blackened? How about the paint around the bumper.
6. For starters I would run Honda Top end cleaner though the motor to clean it out good and blow the crap out. Then go from there.

So pick your poison and think what you think it might be with the new info. provided. Everything listed is a possibility.