Exhaust Tuning - getting rid of the boom

I have a 2001 GTV V6 3.0 24v - it's the CF3 engine. It was fitted with an Autodelta DTM (quad pipes) exhaust by the previous owner. This comes in two versions - mine's the loud version. This makes a great sound especially at the top end of the rev range and also on the overrun, but has always been a little loud in the low to mid rev-range. Especially between 2,200 and 3,000 rpm it's noisy and going up hill in 6th gear often requires a change-down to 5th or 4th just to get the volume down.
So, I was happy enough with this until I decided to put a decat pipe in. As you may all know (I didn't!) catalytic converters are a great way of reducing exhaust noise. Having bought it and fitted it I find the noise between 2,200 and 3,000 rpm almost unbearable ... but the extra power and the extra responsiveness is addictive and I am loathe to reinstate the cat unless I have to.

So questions:
1) why is the the exhaust so loud in the 2,200 to 3,00 rpm range? Obviously it is a question of resonsant frequency but is that a factor of the 24v 3.0 V6 or the length of the exhaust pipes?
2) what can be done to either change the resonant frequency or reduce the volume? One option is of course to reinstate the cat, but I am looking for alternative ideas as even reinstating the cat will only dampen the noise down, not eliminate the problem?
3) I was thinking of going to Powerflow. If I do then can they add wadding or similar to existing exhaust parts or can they only put in silencing/damping into new parts?
4) any other suggestions or advice?

Cheers

Richard

Best bet would be to enlarge the centre muffler (assuming there is one). the bigger the muffler, the less noise you get.

Originally Posted by Pokinacha

Best bet would be to enlarge the centre muffler (assuming there is one). the bigger the muffler, the less noise you get.

I doubt he's got a centre silencer (muffler). I would imagine there's a straight pipe instead of a cat, a straight pipe instead of a centre silencer, and then a silenced backbox on the end.

In this situation, I would replace the de-cat pipe with a silencer. Hopefully you won't lose too much of the crispness that way.

Get the Ragazzon small silencer centre section, you will not lose any performance and it has will get rid of the boom, quite cheap too, try Alfisti.net

Sorry, I should have made things clearer. Present system has original OEM downpipes and flexis, Alfaholics decat, OEM middle section silencer, Autodelta DTM loud-version rear section (no silencers).
I think probably I need to increase silencing in the decat section, but it is not so easy to get a replacement part for that as it also accommodates the pair of lambda sensors.
Any further comments?