fantom alfa

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