I went for the A pillars with the tweeters so they fire straight at each other. Without any obstruction ( this is important if you are also playing with time allignment ). The side view mirror pods which are part of the main door panel would have a good fit too but you'd have the steering obstructing the rhd tweeter and there isn't enough space back there since the Helix tweeter is pretty deep.
An offaxis placement would have been possible had I placed the tweeters on the dash with the supplied pods ( not my thing since those look ugly ), or fiberglassing the a pillars ( which we didn't do at the time and will do in the near future )
Regarding your pre, I didn't understand what really was the question? If the pre is Spotify compatible, you should be able to browse through your own playlists. Check the user manual
Regarding sound deadening, I mentioned or have mentioned multiple times that it's a 3 stage process. The Dampening sheet, MLV and CCF. ALTO is a resonce box. The build quality is pathetic. I've mentioned this too as to why we covered almost all the panels with dampening sheets. The results alone with that were night and day. Has it soundproofed the car? No. Any person who has this knowledge will know. I know it will take another few bulk packs of mlv and ccf to get the maximum out of the car
However, cld can be used on the inner and outer panels of all the metal surfaces on the car as well as the inside of the plastic door panels be it in large pieces or small. The job is to add mass to reduce the resonance and vibration of any panel ( you want to do both the inside and outside metal panels on the doors to reduce the sound from your speakers bouncing off the metal). The door trim pieces on the alto are a flimsy piece of garbage. Even the door clips were swapped on this car so they wouldn't rattle. The next stage is stuffing the hollow bits with ccf which I already have ( Steg )
Btw, I'm glad you questioned this. I've been sound deadening cars since 2012 and started learning all these things back then. It's why I switched to sql and now sq setups from the good old spl days. Its a more tiring process but fun
About your last 2 questions, can I ask which city you live in?
And which cables are you referring to?