My clothes do not seem as fresh as usual and some even smell damp rather than washed (some feel a bit dry too). The soap unit has gone really black from the back and the pipe underneath it is black from the inside too. Please advise.

Not the machine, stop using liquid soap. put machine through a cleaning cycle using a product like affresh.

If you don't leave the door open after each use you will get a musty smell and mold will build up in the bellows. also you need to wipe the bellows dry and the door glass dry as to not get a build up of soap residue on it which will eventually make the door leak. Also, only use one or two tablespoons of soap, not what the soap manufacturer says on the package.

on the front load machines there is a large rubber door boot which gets built up with soap scum. always leave the door open a little bit when the wash day is done. there is a product available called "Afresh". it comes in a pkg with 3 tablets. if the odor is strong follow the directions and use all three tablets. then for maintenance use one tablet a month in an empty machine. what also helps is to make sure you do at least one hot water only wash per month, this helps to disolve all the soap that gets built up in the dispenser tubes and drain hoses etc. hope this helps.

try doing a large load of white, lots of bleach, hot wash and hot rinse, heavy duty cycle

For low water volume - or "Trickling" effect...
1. Turn off the water supply to both hoses.
2. Remove hoses (one at a time so you don't get them confused)
3. On the washer side - look for a removeable screen inside wher you just removed the hose.
4. Using a small screwdriver - pop out the screen and reverse-flush it/clean it. It will likely be full of sediment and calcium deposits from your pipes.
5. Replace the inlet screens.
6. Re-fit the hose to the machine.
7. Turn on water supply valves.
8. re-try.

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Put a half a cup of vinegar into wash load, with soap. It removes the smells.

It sounds like a build up of washing residue. Clean inside the door seal and do a very hot wash with a bit of washing powder and no clothes. Repeat this once a week until smell has gone.

washers and dryers dont make smells themselves its a problem with something u might be putting in it. if u tend to add alot more detergent then usual to your towels then they never completely rinse the water out and will have a mildew smell