What is going wrong with this Espresso shot? I am a coffee roaster and am working through building a standard espresso profile for coffee shops in my area.  I'm working with a particular shop that is interested in my coffee, but for some reason the espresso pulls extremely sour on their machine.  I'm looking for ideas as to what might be going on.  I've done quite a bit of research, so I'll add in what I have done thus far.

My Setup (Shots pull good)


18g grounds, pulling 2 fl oz with a final weight around 50g
Machine is a Rocket Apartamento, nearly brand new.  Water is filtered, non RO, boiler pressure is at 1.2-1.3 bar.
Grinder is a Breville Dose Control Pro, also new.
Shots are pulling in about 26-28 seconds


Results:

Espresso has good crema that stands up for longer than 1 minute and has rich orange / red tones.  Shot is lively, but not sour with no hint of bitterness, and good heavy body.  This isn't a fruit bomb blend or anything, I am more looking for something that a typical coffee shop customer would find palatable.  Espresso stands up to milk very well.  Personally, I almost feel like the roast may be slightly too dark since with milk, I start getting a hint of roasty-ness.

Shop Setup (Sour Shots)


18g grounds, pulling 36g output (supposedly this is 2 fl oz also, can't verify as I haven't seen markings on the glass they are pulling into.  If anything, I would say it is probably a 2.5-3oz pull)
Machine is A Nuevo Simonelli Aurelia 2.  Boiler pressure 1.4 bar, pump pressure 9.5 to 10 later in the pull. Water is not filtered to my knowledge.
Grinder is a used Mazzer, unknown burr wear (personally I'm thinking this is a likely culprit?)
Shots are timed for a 27-30 second pull.


Results:

Shots pull extremely bright / sour, crema tends to be closer to a dirty brown color.  This shot might blend with milk and be drinkable, but just barely.

Confusing points for me that may bear some explaination...


Longer pull (and thus probably a finer grind) results in sour coffee, not bitter.
Higher boiler pressure = higher water temp which tends to scorch if too high and impart more bitter qualities, yet this is sour.
Why is his 2 fl oz of extract such lower weight than my 2 fl oz.  Crema production is heavy on both machines, but the weight difference almost seems to indicate that the crema on his machine is double or more than what my machine is producing which isn't the case.


Troubleshooting espresso via web forum is pretty difficult but I would appreciate any thoughts on how I might go about getting good shots out of this shop machine.  Oh, btw, it wasn't me pulling the shots at the shop, so tamp could definitely be different between the two shots, but given that the pull times are pretty close together, and I did watch the tamp, I can say that technique is at the very minimum similar.  I can say that both tamps appear to at least be even on a naked portafilter.
I don't think anyone can really give a definitive answer to this but I'd like to chime in to try to help. It's almost impossible to find your answer because of how many different variables are at play here. You should first focus on minimising the variance in your methods.


You describe the shop shot as being a 'longer pull' and thereby expecting more bitter results. The problem with that assumption is that you can only truly compare shot times if you have extracted the same output weight in both cases. Since you have not, you have not only extracted much less in your shop shot, but, like you noted, it also took longer. I normally only ever measure my shots by weight because that's the number that tells me how much water got through and thereby how much real contact and subsequent extraction occurred.
I use time as a secondary reference to diagnose other problems in my shot. If I suddenly start extracting a same weight shot in half the time that I normally do, I know something is definitely off. Since your shop shot extracted much less output weight, the sourness is clearly due to some level of underextraction from being stopped too early (relative to your home shot). In any case, I always try to aim for 30g in 30s, which means you probably need to grind slightly finer in the shop to be able to extract more and try to hit the same balance as your home shot.
You mention your water sources being different. This is huge! You will never find the same shot if you are using filtered at home and unfiltered at the shop depending on how hard the water is when unfiltered.


Compare shot times as a secondary to consistent output weight and see how grinding finer in the shop would fare.


Higher boiler pressure does not necessarily point directly towards higher temperatures. Also, higher pressures could impact the puck suddenly when the valve is opened which could crack and cause channeling and therefore underextraction and sourness. Your main issues here are still in point 1.
Weight difference is purely by choice. Crema production is mostly dependent on freshness of beans and less to do with specifics in extraction method. If preset weight quantities are being used as part of a single button-press on a machine, then your problem starts there. You need to weigh every shot and choose when to stop the shot yourself. Without this control, you're simply relinquishing responsibility of the shot and you are at the mercy of the machine, yet another variable to consider in being able to find the right shot.


In conclusion, you MUST minimise variables. Make sure your grind sizes are the same at both locations, ensure your input and output weight are identical, analyse differences in taste and extraction time at same output weight, ensure your sources of water are identical, make sure your tamp pressures are the same. At this point the only real differences will be the machines and then you can start investigating how each machine is affecting the shots. Try to use bottomless portafilters in order to diagnose whether your distribution of your coffee bed was poor and whether channeling occurs during extraction.

Combine all of the parameters above with the taste of your output shots and you'll know the next steps you need to take in finding the problem in your shot.