You might just be hearing the actual engine for the first time
The SSIs and M&K will give you a nearly open path between your ears and the exhaust valves, you will hear much more actual mechanical noise through that setup.
You can adjust the idle back up if you want, changing the exhaust will change your backpressure, and that can chance your idle (and your a/f mixture). Most people probably don't even bother to check or notice.