Help me with smelly laundry.

On heavily soiled loads, I add a scoop or two of washing soda crystals, too.

I really like soda crystals...very cheap and quite useful. 
http://www.dripak.co.uk/
At the end of each month I run my machine on a hot wash with soda crystals. 
Also, I've recently switched to using vinegar with some essential oils (lavender and tea tree) as a fabric softener. 
I buy the vinegar from here:
http://www.summernaturals.co.uk/

I used to find my laundry getting a musty smell if I left it in the machine for just ten minutes or so after the washing cycles finished, back in Austin TX where it's very, very humid and hot much of the year. I attributed it to the humidity and how quickly mold and mildew spores get around in that location -- my washer was in a closet outside on my apartment balcony, so, not air conditioned. Although, an Austin  friend whose washer was inside in the air conditioned house still found this happened to her too unless she made sure to pull it all out the moment the wash had ended.

These days in the UK I still just continue the habit of making sure I don't leave it sitting. That smell is a drag and I've had to wash again so I'd rather be right on it.

The other problem is only washing at 30 degrees or lower. 

Not good for the machine, to be honest, and may not be an adequate temperature to clean the clothes, bedding or towels.

I had this same problem but with my top loader here in the US (it was the towels), I tried white vinegar and it got worse too.
After chatting online to other moms who had this happen, they told me to try and put the towels through a nice hot wash with no detergent, I was very surprised to find my towels were still full of suds, so after a few no soap, hot washes, they smelled nice and fresh. I then ran a load with no clothes and hot water and bleach, in the UK I used to use Miltons from the baby section.
Maybe that'll work for you too.