Actually I disagree. Portable radios don't generally have very wide dynamic range or flat frequency response. They generally have just one signal path, with its response tailored to their built-in amplifier and speaker or speakers. Low-noise, low-distortion circuitry would raise the cost of manufacture significantly, when the market segment that cares about such things is so tiny, it's almost non-existent.
The RF and IF sections of portable radios are also economized to a fare-thee-well. People who need or want high-fidelity FM signals generally use high-fidelity, component FM tuners and receivers. That's what's built for that purpose. That said, I'm sure that some portables are distinctly better than others for this application. If a radio has a line output distinct from its headphone output, that might be a useful hint (or not).
I don't have any specific recommendations because I haven't tested anything like this in decades, but maybe someone else here has. Tandberg used to make a high-quality portable FM radio; I wonder how the Tivoli units measure, for example, but I wouldn't be at all surprised if they have boosted low and high frequencies and a rather sharp cutoff below the range of their built-in speakers, and no greater dynamic range than is needed to support the amplifier power that the unit actually has.
All told I would probably look for a component tuner that has a DC power input, rather than a radio that has its own power amp and speaker or speakers.