Ryan Audio > Loud (never heard of them)

At bakit naman bawal butasan kung bago? That's up to the owner na. Personally, ako ipapabutas ko na siya to achieve good audio quality. If you're worried about your electrical warranty, don't install any ICE until the warranty period is gone. Since magpapainstall ka, I'd say do it whole-heartedly na rin.

A 4-inch speaker isn't what we call a midbass anymore. Midrange na yan as you can't cross it even at 100hz. I'd say you'll be crossing it at around 200-350hz siguro. Heck, Focal's Polyglass lineup has a 4" midrange (VR3) while my midrange is 3". What people normally perceives as bass is at 100hz.

Iba parin ang separates because allowing the tweeter to be placed higher (dashboard versus door placement) allows a heightened soundstage and, for most people, ayun agad maririnig nila. Rather than the singer coming from your legs.. at least nasa head-level mo na. If you want to test it, I assume you have a PC speakers at home. Try placing it near your legs then listen to an acoustic track. Next, you place the speakers in your desk then listen to the same track. More or less you should know what you want ;)