i have some of these
in my front doors, and the midbass doesnt fit unless you cut the hard plastic of the doorcard mount a bit, however, these alpine 6.5"
fit without trimming anything.
alpine type-S speakers are pretty good value, and a good upgrade from factory.
their type-E speakers are nothing special, and not worth it IMO (subjective).
because YOU sit in the front, it's best to focus on upgrading the front speakers to the best you can afford. 'component' speakers have the advantage that you can separately mount and angle the tweeter to achieve a better 'soundstage': imaging, stageheight and width. granted, these are somewhat technical aspects associated with more advanced setups, but most people can hear the difference. it's about creating a lifelike 3-dimensional sound, and not just sound from 'down there in the doors'. coaxials rely on the tweeter being down in the doors, firing straight across at your legs, and not your ears.
aftermarket tweeters will usually be bigger than the factory ones though. they may/may not fit behind the original grills without mods, so you may be required to fit them elsewhere like at the base of the A-pillar or dashtop in the corners there. this will pose cosmetic issues to some people.
i don't rate the 156 rear door speaker position at all. they are quite close to your ears; even closer than the front speakers! as such, if they are audible, then it will drag the soundstage 'backwards' and 'downwards'. unless you wish to cater for rear passengers, i recommending 'fading' to the front so you can't hear them, or disconnecting them altogether (i did, and mounted rear parcel shelf speakers instead). but as such, i don't see a lot of point in spending money to upgrade them, when you could spend that money on better front speakers instead.
if your budget for front speakers is over about AUS$250/UK$125, then i recommend component speakers. if less, you then start to compromise on their quality for the sake of having components; so you can be better off with quality coaxials and accept the compromise these offer (lower stageheight, but improved woofer/tweeter).
ive bought a panasonic head unit which is 4x50w i presume these speakers will be fine with that
remember, this is 'peak' power. in general, best to check the 'RMS' (root mean square) power, which reflects the 'continuous' power output. of course, headunits are always advertised in terms of peak power cos the numbers look impressive. but a 4x50W headunit will be around 4x 18~20WRMS. and there is a lack of bass/treble and higher distortion. you can only expect this, cos the little amplifier ICs inside the headunit are tiny. a decent amplifier is bigger than the entire headunit - for good reason!
anyway, the point being, the power from a headunit is low. it lacks bass/treble and clarity. another good upgrade is adding an amplifier at some stage, especially if you have good quality speakers - they won't ever sound their best without decent amplification.
an amp also provides a (highpass) crossover filter to filter away the subbass. 6.5" speakers - even the best of them - cannot play subbass loudly, and will distort prematurely because of cone overexcursion. just like the factory speakers. if you happened to remove the subbass, factory speakers will even perform much better.
i suppose the message here is that there's no point overcapitilising on your speakers too, unless you plan to add an amplifier. so it probably isn't worth spending AUS$300/UK$150 on speakers if you don't plan to add an amp.
i wouldnt have a clue which speaker brand to choose never felt the need to do anything about a car stereo before.
as much as we love to offer suggestions, you are best to go and audition. speakers vary greatly in sound quality. and often the mainstream brands (alpine, pioneer, kenwood, etc) are NOT the best sounding...by far. they can be good, but not best. it's easy to make 'good' car audio, that is decent and reliable. but takes specialised knowledge/R&D and execution to make them really good. and there ARE really good speakers out there for not too much $$.
so, like buying a car, 'test drive' before buying. see if you can find brands like: boston acoustics, focal, hertz, morel, mb quart, etc. specialist speaker brands. i doubt you'll find these at halfords!
you have to cut the plastic where doorcard mounts, as the doorcard mounts the the door behind the speaker, and then do a bit of shoving.
the front and rear doors are similar. i have these pics from my old 156 and the rear doors:
*plastic speaker 'mount' incorporated (moulded) into door trim (inset), and door trim removed: http://pic20.picturetrail.com/VOL97/...4/12783104.jpg
*back of door trim (paint-on sound deadener) and speaker mount in place: http://pic20.picturetrail.com/VOL97/...4/12847026.jpg
*plastic speaker mount totally cut out to fit new mdf wood spacer ring and speaker directly to door (i subsequently removed these rear speakers for reasons explained above): http://pic20.picturetrail.com/VOL97/...4/12847166.jpg
*rear door trim refitted and new speaker: http://pic20.picturetrail.com/VOL97/...4/12847519.jpg
*front doors sound deadened and sealed with new speaker: http://pic20.picturetrail.com/VOL97/...4/15363285.jpg
the 156 doors are a bit of a pain. as the others mentioned, new speakers don't necessarily fit because of the shape of the plastic mount. this may require trimming (without needing to go to the extreme of cutting it right out!). hence, whilst a so-called 6.5" speaker can be the right diameter, and whilst there's plenty of depth available (without back of speaker hitting window), the space for the magnet/frame is limited. which goes right back to my early post about being mindful of test-fitting before buying, unless you're prepared to do mods. higher quality speakers tend to be harder to fit, cos they tend to have bigger/beefier magnets and voicecoils, and associated bigger frames.
good luck. hope this helps.