juanjo wrote:the biggest plus of the DB is that you can switch off one boiler..
No.
The biggest plus of the DB design is that brewing is independent of steaming. Steam requires superheated water. If the brew water is drawn from, or through, the same boiler that produces steam, the water is much too hot for espresso and needs to be cooled, using a large brass heat-sink. With a DB, the brew water can be heated to the desired brew temp (or just a couple of degrees above it to compensate for loss) and so there is no need for a heat-sink. The lack of a heat sink and the typically smaller volume of the brew kettle on a DB means the temperature can be changed readily, because there is no large thermal mass to deal with. Changes can be precise, too, whereas with an HX, whose temperature is regulated by flushing, there is far less precision, and moreover, the HX is only at that desired temperature range after flushing for a brief time, as it will quickly rebound.