@mohaveblues I think there’s a typo, the H 9 is mostly identical with the B 291 (not 297). My favourite was always the B 795, but couldn’t afford it back then ;-)
My instant reaction was that the B 291 doesn’t have ’a phone preamp’, in the sense of our usual RIAA preamps and such. Just looked up the Service Manual on Vinyl Engine. It confirms that the B 291 doesn’t have a ’typical’ phono preamp, yet there’s a section that talks about a preamplifier. It clarifies that the B 291 base model does have a linear MM amplifier, which works as an impedance converter. It can also been seen in the schematic. The manual explicitly states that the B 291 for this reason has been designed to work with MM cartridges (P Mount systems, the manual also includes a list of tested and recommended alternative cartridges).
In addition, an RIAA module was planned that, according to the manual, could be plugged onto the PCB. The manual also explains how this activity is to be performed, but I’m not sure whether such a module ever was available (the schematic on that page in the manual is hand drawn...)
So in short: yes the B291 does have a preamp section, but not a full one with RIAA EQ, unless the extra module has been added. To disconnect that, you need to locate the components on the PCB and bypass them somehow. A bit of a surgery, I suppose.
(Side remark: the service manual is in German, so I have translated that part. I’m German, but @tablejockey Revox isn’t, they’re Swiss 😉)