Given your background with PIDs, I assume you've already tried tweaking the brew temperature?
Do you have any way to measure temperature stability during the shot? I know the E61 HX machines are pretty sensitive to how much water you flush through before the shot -- but not quite sure how the DB version is going to behave.
My own suspicion is that peoples' tolerance to acidity varies quite a bit. I'm not particularly keen on it myself: Has Bean's "Blake" blend (which I still rate really highly) is about as far as I tend to go for espresso.
I did try upping the temp to see if that was causing the sourness. Other people report having to updose with these light coffees, but I found that made it immeasureably worse. I found a 14g double pulled to about 2oz gave a reasonable shot, but quite thin...certainly not the thick gloopy stuff I normally like. As for temp stability, I have no way of measuring it, but other coffees pull very nicely, shot after shot.
The more I read about this trend for light roasts the more I wonder if there isn't a touch of the Emporer's New Clothes about it. Certainly the UK proponents are totally apeing what is going on in the US, and as we know, they are always looking for 'something new'. Roasteries seem to be following the pattern of micro-breweries, which is a good thing, but there is a great deal of marketing involved. If you read the 'dogma' about the so-called 'third wave' roasteries, they are trying to claim a sort of 'appellation', whereby they 'work closely' with coffee growers and estates, often making allusions to giving the farmers a fairer deal. What this means is that you get the obligatory photo gallery on their websites with their holiday shots in Central America or Africa.
Back to the SM coffee I had, one of the bags was called 'Guji Espresso'. What it was, was very good quality Yirgacheffe, that prepared in a filter with a comparatively low dose was absolutely lovely, but espresso? No, sorry, it just isn't.
This is the thing, in this drive to get more and more flavour from these coffees by up-dosing massively and putting them through an espresso machine with a tiny output of liquid they are missing the point that sometimes less is more, and the subtleties are easily lost. Espresso is not the only way to prepare coffee, but this deconstructed Starbuck's generation can't quite see beyond it yet.
I predict the next fashionable thing will be filter coffee
(this all said, I'm still intrigued enough to be thinking about going to Prufrocks or one of those other wanky London places, even though it means sitting in a room full of Nathan Barleys)
