For comparison, can anyone give me the diameter of the hole area of the HQ14? Height seems to be 24mm according to EP.

I just wanted to report back after your help on this subject. Basically all my shots with dark roasts and also medium roasts tasted bitter. I tried up-dosing and flushing the group for up to 10 sec. Both didn't have any significant effect on my set-up. Playing with shot-flow didn't help either. I then installed a group-head PID. This made quite a difference. At the moment I am mainly using medium roasted classical Italian espresso blends (Danesi, Kimbo, Lavazza etc.). Taste in most cases is best with PID set to 85C (which does not refer to the actual brew temp. that is higher), no flush. The original thermostat was set at 95C so I was very probably burning my coffee. Not sure if this is a particular problem of the 230V machines. Taste was still a bit harsh. I tried stopping the pump by releasing the lever after about 7s to reduce preinfusion pressure. This seemed to have a slight positive effect. I then decided to install a dimmer on the pump. Following TomC's recommendations in one of the Strega threads I turn the dimmer down as far as it will go without stalling the pump. I now preinfuse for 12- 14 sec. Finally I am relatively constantly pulling good shots, the best shots so fare. I can now concentrate on dialing in and differences between blends and beans. Thanks for your help!

Those blends probably all have some Robusta? Often tastes burnt. There are some great all-Arabica dark roasts you can access easily here in Europe. Try Passalacqua from Napoli for starters. It will blow you mind.
PC

Yes, I usually try an avoid blends with high robusta content for that reason. The blend I'm using at the moment is an all-arabica: Kimbo Gold. Unfortunately it's a bit on the old side, but still tasty in a cappuccino. Thank you for your recommendation. I will definitely try and get hold of some.

Ravco,do you also have an external sensor on the strega grouphead? What does that read?
I set the pid (basic auber setup) on my strega to 89°C for darker roasts and to 91-93°C for lighter roasts.
But the strega tends to keep getting warmer , so after warming up for 3/4th of an hour I need to lower those pid settings a bit.

If you tend to get bitter tasting coffee, maybe try to coarsen the grind, that helps for me.

No, I don't have an external temp. sensor on the group head. Doesn't seem to have any relevance for me, as brew-temp. is also determined by the water exiting the group-head. I tried grinding coarser on my original setup, but it didn't seem to have any (not enough) effect. On my new set-up with PID and dimmer bitterness is greatly reduced so that I can detect the sweet spot. For the blends I'm using at the moment it is mostly associated with a grind setting resulting in a slightly up-dosed basket (9-10g in a 7-9g LM single basket).