Simplest way to get a bit more 'growl' is to drill some holes in the lower half of the OE airbox on the right side as you look at the engine. Add an ITG/K&N/Pipercross panel filter for a few 'claimed' BHP (not convinced but they do sound good!).
The Fatty setup, and variants thereof, is probably the next step and a nice upgrade both visually and for the noise. The KTR induction kit is more or less the same principle as the Fatty but more £s and quite a bit louder (in my experience).
I tried all of the above before settling on the BMC CDA Carbon airbox, which is similar size/position to the Ph1 airbox. I've got mine coupled to a Samxo silicone elbow. The BMC is hideously expensive new but they do pop up now and again second-hand. Great noise, not too loud or harsh, look great and cant be doing any harm to the engine breathing/power.
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