I just plumbed in my Izzo Alex Duetto, with a water filter system from Chris Coffee. The pump pressure gauge now reads about 4.5bar just with line pressure. I adjusted pump pressure during brewing to 9bar. 4.5bar seemed high to me, so I talked with tech support at Chris Coffee this morning, and he told me it's not a problem - they've seen as high as 6bar or so. He said as long as the brew pressure is about 9bar, it should be fine, but I can add a pressure regulator if I want to.

Does anyone have a different opinion about whether a pressure regulator is necessary?

For sure, CC is giving you good advice exactly as they say it - "no problem". A better way to check line pressure the machine sees is to electrically disconnect the pump (simple) and brew away with an empty portafilter. What does the gage read now?

If it were me, I would install one of their (Chris' Coffee) pressure regulators and adjust same to produce 2.50 bar +/- 0.25 under the conditions detailed above. Also install a shut-off valve (ball valve) as close to the machine as is reasonable. Now adjust your pump to produce 9.0 bar +0/-0.25 and you will be a happy barista.

What is the part that is most likely to feel the 4.5bar?

It's the autofill solenoid.

The Izzo Alex Duetto II's I've found online seem to use a Sirai valve.
Look up the part:
http://www.sirai.com/inglese/pdf/catL.pdf
It'll likely handle 20, 30, or even 40 bar.

Now ask, if the valve failed open, what's next in line?

The boiler.

If the safety didn't shut off, you'd be over pressuring your boiler pretty severely.
Sure it requires a double failure, but a 2.5 bar regulator acts as a third layer of safety.

Thanks for the reply Eric. I'm guessing that you're thinking if I disconnect the pump and brew, I might see a lower "actual" line pressure? Which I could then adjust with a pressure regulator. Since I've already set the pump pressure to 9bar, would knowing and adjusting the actual line pressure make any difference in the cup? Or is it simply a safety issue as allon suggests?

With a rotary pump, isn't a regulator recommended to avoid pressure fluctuations?

Depends on the pump type. A balanced bypass rotary will keep an even output pressure with fluctuation in line pressure but a standard will fluctuate with the line pressure giving you erratic brew pressures. I run my machine at around 2.8 bar mains with a regulator before it. My machine specs a 4 bar input max.

I'm guessing that you're thinking if I disconnect the pump and brew, I might see a lower "actual" line pressure?

I'm saying that you will see a more realistic line pressure which may also be 4.5 bar (or not) depending upon what is between your machine and the main water line inside the house. I acknowledge your filtration system but there can be twists and turns, supply line sizes, elbows, etc., etc.

Your pump is fitted with a balanced by-pass valve, typical of the majority of high-end home machines and not so typical of commercial machines. That's really handy (and necessary) on your machine because users can switch between reservoir supply or mains supply at the twist of the wrist. This feature is far more tolerant of varying inlet pressures than a pump not so equipped.

Your're in sorta a grey area in that if you had said 6.5 bar instead of 4.5 bar, the response from CC would likely have been different and the response from others would have been the name/address of the nearest pressure regulator supplier . Given your attention to detail re pressure adjustments, there will be no difference "in the cup" but I still stick with the original recommendation.

I've got at least 6 bar line pressure at home. Once my pump failed on the Marzocco, but I didn't really notice it for a few weeks....

Showers are great, too!