The calender function is completely worth giving up in order to go with an aftermarket radio. Who even needs a calender in their car? Any basic phone has a calender.
Those line out converters are limited by the quality of the source, being the stock radio, and no matter how flat you tune the stock EQ, it never gives you quite as clean/flat as a signal as some would like. Don't get me wrong though, it won't sound any worse, just not quite as good as an aftermarket radio with dedicated line-outs. The Monsoon could be different since it's internals are better, but then money comes into play. It will save you a lot to avoid the Monsoon regardless of what you wind up doing.
Hell, even try some speakers alone and see how it sounds, ones with a 2-ohm resistance come to mind to get as much power as head units offer. My Infinity Kappa's kept me content running off my head unit until I had cash for an amplifier. The Infinity Reference are damn cheap for how loud they can be.
Subwoofers are another game. If you just want to be able to hear the low end and get a little more bump, then those bass tubes are a good deal and very simple to wire. If you want audiophile sound quality and/or serious bass, you really need an actual box sized and tuned to the sub.
