I'm with the rest of everyone, i'd definately say the powervalves are set incorrectly. Especially since it just had a re-build, chances are that the powervalves re re-installed in an incorrect position. Talk to your local motorbike shop, they should be able to get their hands on a manual for you and it'll tell you exactly in what position your PV's should be when they are installed.
As for the whole powerband thing...it is such a misconception about what it is. A powerband is not an accessory you can add to the bike, its not a fantastic added feature of a 2 stroke and a motor sure as hell can't run without one! All the powerband is is the rev range where the bike makes the best power. 4 Strokes have got a powerband also, its just not as noticible as on a 2 stroke. You can feel the powerband kick in, its where the bike starts to pull a lot harder, on a 4 stroke it is harder to feel but ride a YZ450 or CRF450 and you'll sure as hell feel a big change when the bike starts to rev up, it'll try and pull your arms off.
The only reason 2 strokes have a more noticable powerband is because of the expansion chamber. A 2 stroke is essentially being supercharged by the exhaust pulses. When the exhaust gas exits the cylinder via the exhaust port, it creates a pressure wave. This pressure wave expands through the expansion chamber and then contracts again as the expansion chamber narrows at the end of it. The pulse is then shot back towards the exhaust port and a causes the fuel that has passed straight across the inlet port and into the exhaust port to get pushed back into the combustion chamber. The piston at this point has covered the inlet port and hence the pressure wave has actually added more fuel into the combustion chamber and also partially presurised it.
For this reason, when your nice pretty FMF expansion chamber gets a big dent in the side of it you will notice a vast difference in how the bike performs. The expansion chamber is no longer pushing as much fuel/pressure back into the combustion chamber hence the bike wont run as well.
Different brand pipes will have different effects on the engine. Your stock pipe is tuned to provide good performance all round. An FMF might be a different shape/length/size so it might add some extra kick at top end. A Pro Circuit pipe for the same bike might actually add more power to the bottom end because it has a different shape/length/size.
It is my understanding that the powervalve somewhat stops the fuel from entering the exhaust port by partially shutting it off to flow, but it still lets the exhaust gas come back and presurise the cylinder hence more throroughly filling the cylinder with gas and creating a bigger bang!! Please feel free to correct me on this if it is not correct.
This presurising effect as i said earlier is essentially supercharging the cylinder. Some people say that it is more like a turbo and that is why you get a big kick of power when the bike hits its powerband but i'm not going to agree or disagree with that.
The same thing can not be said with 4 strokes because they have valves that stop the fuel/air mix going straight out the exhaust. Mind you if you've got a really lumpy cam then chance are you might be loosing a bit of fuel out the exhaust and hence shooting some flames out your mufler (ie. V8 supercars). This is why 4 strokes do not have expansion chambers, merely just a bit of tube to pass the exhaust gas through.
I'm sorry to rant and rave about this but i see so many people talking about powerband and thinking that its a super special thing but its not. It is just another characteristic of the engine!
Good luck getting your RMX running properly mate, they're a good fun bike to ride.
Matty
