I have a question about DB performance when there is an HX preheat of the incoming water via the steam boiler, and that boiler can be switched off, as in the Alex Duetto. Has anyone compared the temperature stability with the steam boiler switched off and on? Seems like that would present some potential issues, maybe only in recovery time for the brew boiler since it's only a couple ounces of water in a .8 liter boiler.

[edit] - Just remembered the Bella Barista review, and it specifies that recovery time is the compromise.

I haven't played around with thermometer portafilter yet, but AFAIK the PID typically shows a drop of two degrees after pulling a shot. With group smoothing out temperature fluctuations there should be not much of a compromise in temperature stability. But if you are going to pull a lot of shots in a row, then it will start to make a difference. So according to me experience disabling preheat will slow down recovery, but will not make much of a difference when pulling a single shot.

This should answer the question:

http://www.home-barista.com/espresso-machines/temperature-study-of-alex-duetto-t7950.html

Thanks, there was something rattling around the back of my head I couldn't put my finger on. I read that thread some months ago.