I recently pulled my Isomac e61 machine apart to tweak the brew pressure, and figured i would take a photo while i had the case off.

On an e-61 machine with a tank and vibration pump such as this one (as opposed to a bigger and more expensive, plumbed in rotary pump), brew pressure is controlled by the over-pressure valve. Since a vibration pump is powerful enough to exceed 15 bar (and we only want around 9 bar for espresso), the OPV bleeds off pressure in real time, mechanically, while the brew lever is in the up position.

You can set the point at which it bleeds off pressure by simply turning the OPV one way or the other. On my machine, a counter-clockwise quarter turn was enough to reduce the brew pressure to 9 bar (from about 9.7 bar). This has slightly improved the flavour of the coffee - perhaps it is extracting less of the bitters and more of the good stuff. Taste is definitely more rounded. Everyone seems to like the taste of shots at 9bar, so i'm sticking to that for now.

With less brew pressure you also get a slightly slower flow rate, which on an e61 with only a few seconds of pre-infusion, probably helps as you can stretch your shots out that little bit longer and end up with the same weight/volume in the shot. But less pressure also means less "stuff" extracted from the coffee.

The other interesting thing that can be adjusted (although i haven't had to adjust mine) is the pressurestat. This is basically the thing that detects pressure in the boiler, and sends a signal to the control board to tell the element to turn on or off. My pressurestat keeps the boiler between 1.0 and 1.3 bar, which is pretty standard. In a single boiler heat exchanger, that's a happy medium - anything more and you'd need a much longer HX pipe or it'd run too hot, anything less than 1bar and you wouldn't have enough pressure for getting that perfect microfoam.

Machines like the Expobar Minore with dual boilers have the advantage that their steam boiler can simply sit at 1.5bar all the time, giving great steam pressure while the brew boiler sits at just the right pressure to deliver the required brew water temp. 

One disadvantage of dual boilers is your brew water comes from inside a boiler which has an element exposed to the brew water. Unless you filter your water well (think reverse osmosis or ion-exchange resin) that boiler may have scale, which can affect the taste of water. HX brew water comes through the HX pipe from the tank, so there's less opportunity for scaley tasting water getting into your coffee.

The thing i love about e61 machines is, parts are so readily available and can be interchanged easily - i could upgrade my pressurestat to a more expensive brand/model and reduce the deadband if i wanted. Elements can be changed, silicon tubing can be replaced with ease, nothing is proprietary. Everything is pretty solid and although not every e61 is the same, most will pull a very respectable shot.