I have 2 pounds dark DME, and 4 pounds liquid dark ME.
and 2 oz of cascade hops all left over from previous batches.
what would be a good way to use all these ingredients together in a future batch? any grain suggestions?
You could use what you've got, maybe add a little crystal malt as a steeping grain to add some body, divide the cascades over three additions at 60, 20 and 5. Use WLP001 or Safale US-05 yeast. Oxymoron beer - a dark pale ale!
On another note, if you don't use dark extract, but only light, then you can get the darkness and flavor you need for dark beers fron your speciality steeping grains. The problem with the dark extract is that you don't know what's in it so you don't know what to add. IMHO. Plus, the light is more versatile and can be used to brew any style of beer, including stouts and porters.