2006 Ram 1500ST ; scoping out new system / 2 questions

Hi forum, I am new to this forum and a owner of a new to me 2006 Ram 1500ST. I'm an old head (57) and have not had a decent stereo in any of my cars for at least 30yrs due to life in general. Now, I want to treat myself to a decent (not window shattering) system. Went to a local shop and walked in and told the young man behind the counter what I had, and that I was interested in putting a system in it. First reply was "do you want 2x10's or 2x12's"? I said, "a new WHOLE system (head unit, door speakers etc)." Once he grasped what I was really needing I asked for a quote in writing which he gave me; Pioneer head, 2-Infiniti 6x9, 2- Infinity 6" and a 600W 5 ch Infiniti amp because I was not sure if I wanted to explore a single sub later or not, but would at least have the correct amp and wiring in place later if I so desired. FIRST QUESTION; When I asked about the 3 speakers in the dash, he said that they wouldn't replace them unless they were blown or non functional. OK, why not? I already did my research and know that those speakers are up there and if I want the shop to fill them in so I can do this stereo in one swing, that should be my decision and not theirs. Also, they did not have any of the Infiniti systems available for me to hear which also bothered me. I already went to a prior shop and got a comparable quote, but they are WAY on the other end of town. I found this place within walking distance from my apt and decided to check them out. Wanted to get some opinions on the 3 dash speaker setup and if I should bother with them or not. Also a SECOND question; if I do go with a sub, I really do not want to kill off the under rear seat storage with a big box as I am looking to possibly purchase a Husky Gearbox or Weather tech storage box to go under there. Have any of you heard or seen a single sub install in the passenger side with the rest of the area used for the storage box? I'd be willing to cut the storage box down to house the single speaker enclosure. I know this idea sounds crazy but wanted to ask. Thank you for everything.

When I replaced my audio, I went with Infinity 2 way components up front. The 6x9 bass/full range in the door with the tweeters in the left-right dash speaker locations. I put the 6 1/2 Infinity 2 ways in the rear doors, and an Infinity Primus 12" sub in a box in the back. I'm running a 4 channel Infinity amp for the main speakers and a mono infinity amp for the sub.

Stock, the 3 factory dash speakers are wired together. The center speaker takes the positive from one channel and the negative from the other. I did not use those or the wiring. You can't really re-use those speakers since the factory amp is wired with internal crossovers to handle those, unless you are somehow also using the factory amp. So, ask whoever is doing the install how or if they plan to incorporate them if they aren't replacing them.

If I were re-doing mine and had money to burn, I would get 3 way components, put the mids in the dash, and the tweeters in the sail panels in the doors by the sideview mirrors. I don't miss dedicated mids at all, it would be more for completeness and OCD than sound quality. But who knows, maybe it would sound even better. I figured in a diesel truck, the added expense for going to 3 way components would not pay off. Mids and 3-way crossovers were double what I was spending on the 2-way components at the time.

I can't help with question #2.

I used Infinity Kappa speakers when doing my stereo.
3 1/2 in the dash.
6x9 three way in the front and 6 1/2 component in the rear.
On the 6x9, I had to slice around the opening and tap the sheet metal to clear the speaker frame. This was easier than I thought and no big deal.

The two dash speakers I put high pass filters on and wired them with the front speakers. The center I just left in place, it's tiny.

Amp and sub I put under the rear seat.
I'm pretty sure you can buy something that will put the sub under the center console where the middle seat lifts up and you can move the jack under the passenger seat for the amp.