Byhoo5 the best sounding exhaust i've heard by far, is the Carbon fibre-canned H-Bomb... if you're lucky and ask around, you might find one... they offer a very deep note, without being TOO loud...
Aside from that, changing your muffler is the only way to attain a deeper exhaust note. repacking the muffler loosely is no good, as it will just blow the packing out quicker...
KDF on egay sell the cheapest big bore muffler pipes... bout $100 posted....
If you've had a look at my Thumpy build thread, the muffler on that bike is the KDF big bore exhaust..