I have also just purchased a Greyflex 13ft 9/10 but I have yet to cast a line with it.
The rod feels quite stiff so you would probably be best following G Ritchies advice and go for a slightly heavier line.
I would be wary of getting a short headed spey line as you may find the head is a little too short. Try and have a cast with one first before you splash out a fair chunk of money. I am no expert caster but I find a Rio Mid Spey has just about the ideal length head for myself. Yesterday I was using a Hardy Spey sinking line and found the head was far too short for my liking.