I had a look round yesterday and you are quite correct, there appears to be very few (if any) DAB radios with any sort of tone control at all, I have no idea why this would be the case though, as I can see no reason why a simple tone control (a capacitor and a variable resister connected in series with the speaker output)) couldn't be put on DAB radios.
If the DAB radio has a headphone socket you could always get a cheap (about £5) set of travel speakers and plug those into the dab radio as they would produce less bass.