If my head unit is 4x50w and my speakers are rated 35w input do I NEED an amp?

Lots of people on t'interweb say the amp makes a big difference but in a car that has no space for one is it really needed if all you want to do is listen to decent music at a decent volume – not play tunes that sound like you are sticking your head in a tumble-dryer at full chat.

Head unit (that i'm planning on getting) is a decent Alpine unit (104BTi) and speakers will be basic after-market 'upgrades' over OEM card cones.

I've had both set-ups and sound is better with an amp but easily good enough for me with a decent head unit.

A decent head unit with quality speakers should be fine, although you'll be surprised where you can put an amp in a car; my old Puma had a nice Genesis amp mounted on the floor under the passenger seat, into Infinity Kappa speakers. Sounded lovely. The amp is now on the side of the boot in my Octavia.

If you can afford it (cost & time/effort), then putting sound-deadening on/in the door cards helps too. I basically had the door sealed up using the stuff in an old car of mine and it helped improve the general sound a lot.
I've had a horrendous setup using a full Alpine setup with amp before, on paper it seemed good, but the head-unit was a bit carp and I never managed a decent sound from it – was really 'shrill' sounding. Changed the HU and it improved it – lesson learnt there was to listen to the bits beforehand!

Are Sony HU's any better than Alpine? The two brands have two very similar units for a similar price – which are usually better?

There won't really be anything in it TBH, I wouldn't have thought.

Surely if you run high power head units you should be using speakers with AT LEAST the same maximum power rating, or they will just blow if you crank the volume up. The Headunit rating wattage will be in peak power, but the speakers may be rated RMS. Its not an amp you need but bigger speakers in that case

If my head unit is 4x50w and my speakers are rated 35w input do I NEED an amp?

No you do not need an amp. When you turn an amplifier up to max, they generally send a distorted signal to the speakers, which is how most people blow them, therefore: You should always over power speakers so the amp doesn't send a distorted signal to the speakers.

I have a 75w RMS per channel amp running 40w (i think) speakers. If you wanted to deafen yourself you could turn it up to max and the speakers would not blow. If it was the other way around (low wattage amp, higher wattage speakers) they would probably blow. It's the same for home/pa amp/speakers too.

the reason people use amps is more to do with quality as allthough your head unit has enough power in theory a seperate (quality) amp will have far better headroom and control over the speaker . and as for all the crap about blowing speakers – you would notice the distorted (clipped) sound long before they blew as it becomes very uncomftable listen to . with an amp you could allways add a subwoofer which would take the bass work off your front speakers