[Arran]

My experience with input jacks, usually for phonographs, being added to antique radios is that they usually have hum problems because they people that put them in ignore lead dress, don't use shielding, etc. In fact the aftermarket add on jacks are usually so bad that they are among the first things I remove because of this. Even on sets that were built with phono jacks the volume levels aren't anything spectacular, and the radio is usually louder, with better fidelity. It would sort of make sense since the front end of the radio is acting as a preamplifier for the audio output section, especially on a set with a tuned RF amplifier stage. As for audio quality what you need is a phonograph oscillator/am transmitter with a wider RF bandwidth, some old receivers had variable IF bandwidth for higher fidelity, I don't know if the 690s did or not.
Regards
Arran