To give an idea of the sort of results that can be expected I have recorded a couple of short audio clips playing guitar through a valve radio. The radio is a Bush VHF61 and the guitar is plugged into the gram input. The audio output stage of the Bush is the typical single triode gain stage into a pentode output valve so is a similar topology to the radio that the OP is looking at using. The speaker in the Bush is a relatively small 6x4 elliptical speaker so not ideal but even so I don't think it sounds too bad.
Clip 1 is just the guitar straight into the radio, clip 2 is with an overdrive pedal (Ibanez Tube Screamer) set to boost the signal and overdrive the valves.
http://www.pjdamplifiers.co.uk/media/radio-amp.html