Thanks guys,
it is a specific solution for a specific problem.
This or similar approach can be used for 4 relays attenuator but the relays should be 3 or 4 pole variants which isn't simple anymore. In this type of application, I prefer using reed relays or standard types (with bifurcated contacts). Cheaper reed relays are usually single pole so design mimics a standard switch.
Which type of control switch (or logic) MBB or BBM should be used, depends also of the position of resistors network, not only if it need to keep DC path. Here, for example, BBM should be used because in short time between steps the network goes to max. resistance and the system will go temporary to state of minimal gain which is good. If MBB type is used, in short times between steps, the network goes to lower resistance than adjacent steps are, so the system goes to higher gain in a short time which isn't good. Sometimes a small delay in relay's switching the state caused with a protection diode can make a big problem.
There is a also situations where most part of an attenuator should be MBB and just one or two should be BBM
). One example is the mic/line attenuator in NEVE preamps, and there is a famous OFF position where additional gain stage is inserted in a chain.