So there appears to be a genius, Herbert from HSE, who has developed a DAB+ upgrade for valve radios.
He has designed and makes a custom module which plugs in in place of the mixer valve in a AM radio, converting it to DAB without making any other changes. Simple enough idea, but the complex implementation of it...
The really clever bit is that it scans the DAB bands on startup, while the set warms up, and then spaces the found stations along the dial, so you 'tune' your set as normal!
http://www.hse-radio.de/
Has anyone bought or tried one of these? They are not cheap obviously, (around 132 euro) but are very interesting. Think I might need to try one!
NB - I am in no way affiliated with HSE, just came across them while doing research. I have however emailed Herbert and he will ship to the UK. He builds to order as it obviously depends on the mixer valve.