I have a Lelit espresso machine which came, like nearly all other machines, with multiple baskets. I got a double (up to 20g), a small double (up to 18g) and a single (up to 9g) basket.
The machine lets me brew phenomenally delicious espresso with the large double basket, which has straight walls and lets the water flow through the coffee evenly. The espresso has a really nice texture, it's neither sour nor bitter, perfectly balanced, very sweet and just very enjoyable in general.
The single and the small double baskets however only have holes in the middle and are kind of shaped like a funnel. I understand the purpose of this shape, it's to slow the water down so you get similar extraction times with the same grind setting.
During the past few weeks I tried to use the single baskets exclusively to see if I can make single shots instead of having to drink a doppio every time. I have tried adjusting a lot of factors and could not get an espresso that was as good as with the double basket. I tried adjusting temperature, pump pressure, grind size, tamping pressure, and extraction ratio. I have tried using 7,8, and 9 grams of coffee with a brew ratio of 1.8 to 2.5 and none of them were as good as with the double basket.
No matter what I tried, the espresso always had that channeling taste. It was sour and bitter at the same time. Inspecting the pucks afterwards showed no clear sign of channeling most of the time.
After this frustrating ordeal I just went back to using the double basket, which clearly makes better espresso (I even did a blind test to see if I can tell the difference, I got 4 out of 4 right).
Are single baskets just not made for specialty coffee? Does the funnel shape cause circulation at the edges and therefore higher extraction at the edges compared to the middle of the puck? Is it possible to get an espresso from a single basket that is as delicious as one made with a double basket? If not, can I buy a single basket that has the same shape as a double basket but with room for less coffee? I'm already pretty close to the finest grind setting on my grinder so I'm scared that using less coffee would make it flow faster than I can counteract with grinding finer.