Ok I've played it through.

In general, this can be thought of as a progression in Dminor.

Ok so it has a B in it, which means for the first part of the progression you can probably think of it as D Dorian (relative to the c major scale)
But when you play the Bb chord, its D Aeolian (relative to the F major scale)

The Ab chord doesn't fit at all really. If you like the sound of it, then by all means play it. Here's what I would play (if I were to use the Aflat chord):

//D //B //F //Bb A //
//D //B //F //Ab G //

I think that sounds cooler.
Here's a little mp3 of me playing it which I spent one minute on:

http://www.geocities.com/thejuju2000/Untitled.mp3

sorry it's not that loud.