Again, photos have been very helpful.
It has been suggested to use wickes high tack flashing tape
Ah yes. I have used this in the past as a cheaper alternative. For the purposes of sealing over the holes, it should work ok.
I have found the main limitations can be:
*thinner and more flexi than 'proper' incar sound deadener; the stiffer the better, cos it will not flex, and will help create a more rigid 'enclosure' for the speaker
*narrow rolls: I haven't seen it in sheets, and only in rolls, up to 4" wide; just makes it more tedious to apply
*not as dense/heavy: not that important for the purposes of sealing holes; but the other purpose of sound deadener is to add mass to the panels, to reduce the (audible) resonant frequency; means less audible vibrations/rattles; deadener also converts kinetic energy (vibrations) into thermal energy (heat) as part of the 'stopping vibrations' function
But definitely a reasonable alternative if it does work out cheaper. But could be worth roughly calculating how much it costs for a roll or two, compared with 3 'sheets' of Dynamat Xtreme or alternative incar deadener.
The problem with fitting speakers in the original position is that they get covered by the passenger and my legs,
or making a kick panel box,
I couldn’t find any info on what volume would be good for 6.5" mid woofers. I was thinking the sound stage would be better with the speakers facing front to back, compared to side to side but don't know if the lack of space in the kick panel box compared to the space in the door would justify making them. I could cut the carpet away and leave the back of the kick panel box open, so that sound goes into the space behind it, what do you think?
What about dash mounting?
There are pros and cons to each.
From a sound quality/performance point of view, you should aim to:
*have at least 10L/0.3cuft for a sealed enclosure for the 5.25" woofer
*aim upwards and angled 'inwards' towards the opposite-seat headrest
A door cavity might be around 1cuft/28L of volume, which is fine for a car audio speaker. Speakers are generally designed to be mounted in the door, taking into account the size of the average car door.
If the 'enclosure' is very small, you won't have much midbass. In a door, a 5.25" speaker should be able to play down to 100~150Hz without probs. In something like a 3L/0.1cuft enclosure, it may struggle to play down to 200Hz loudly; the small box overdampens cone movement.
The concept of a 'sealed enclosure' is quite important. In the kickpanel, it can be difficult to build a largish enclosure; but also ensure it's somewhat sealed. You don't want the soundwaves off the front of the cone interacting with the soundwaves off the rear; interaction can lead to cancellation/phase issues, which manifests as loss of some frequencies like loss of bass or some of the midrange/treble (it'll just sound relatively poor).
So it ends up being a compromise. The ideal position might well be in the kickpanel areas, angled appropriately. But build suitable enclosures will be a challenge. As such, the doors remain the most convenient place to install, at some loss of imaging. I think the Alfa 75 set is even worse, yeah?!
I wouldn't mount them on the dash without doing a lot of work. It looks a bit flimsy. You'd really need to reinforce it and build enclosures underneath. That might be do-able, but it's again another inconvenient approach.
I have heard that it’s not good to mount the amps on the box due to vibrations to the pcb. I am happy to mount the amps on the floor, may even light up around them
An amp of good build quality (good soldering) will not fail. They are subject to constant vibration from being inside the car anyway. 'Heat' remains the biggest cause for amplifier failure, but most amps are well designed these days.
But if you can floor mount them, no probs.
The design of an installation is half the fun
. As you're doing, it's good to consider all options and trial-fit things here and there, then decide on what the best compromise is.
If you're going all-out, I'd build kickpanel enclosures for the 5.25" drivers. You can make a complex shape (with mdf wood and fibreglass combo) that houses the speaker in the kicks, and extend the enclosure onto the floor as a low, flat 'box'. You will lose some floor 'height', but this area can be the most convenient to build into.