I'm not sure whether I'm experienced enough to provide real advice re blending but the following has been working for a while for both myself, (when I'm not experimenting), and a few friends.

What started as a basic Mocha/Java based on a blend from my sister who owns a boutique roastery in Cape Town, I switched from using Java (or Colombian) to Brazil and added a dash of PNG. So what started as a 60%/40% Java/Ethiop has become a Brazil - 52%/Ethiopian 35%/PNG - 13%. I am not always quite as precise as that and it's usually 60% Brazil, 40% Ethiop, then a dash of PNG to liven it up a bit.
(Currently using Brazil Pulped Nat, Ethiop Gamb Nat, PNG Wahgi AA)

My beans are predominately sourced from CSs so I leverage on their expert knowledge of bean choice, and roasted on my Hottop to CS9-10. I tend to allow the Ethiopian & PNG a bit darker than the Brazil, although the Ethiop tends to roast a bit unevenly. Used for neat Expresso as well as Milk Based. While I use use a Giotto, my friends have lower end domestic machines and are pretty happy with the results.

For whats it's worth  

Graham

I have in fact just roasted some Eth Harr in place of the Ethiop Gamb, but not yet replaced it in the blend, so not sure whether it will affect it as yet.

What I can say is I have been roasting some Sumatra (Panjang Mandheling), to CS9-10 and having it as a single origin, and found it to be delicious, and on a par if not better than the blend I provided above. So I imagine it would probably add something extra to the blend. I'll give it a go next time.  

I did find that the Sumatra seems to be best when kept for at least a week or so before using.

Graham