If it's wired like the Mullard circuit, I wouldn't expect to see more than a fraction of an ohm between any output tap and chassis. Anything higher would lose a lot of the power output if it was winding resistance!
Don't forget most multimeters have measurable lead resistance (short the leads together, note the reading, then subtract this reading from any subsequent measurement of low resistances).
If it's a different circuit, possibly without any feedback, the resistance might be anything- even open circuit, without it necessarily being a fault.
Well worth checking the continuity of ALL of the output transformer windings, though. The primaries on small p-p transformers like these are not particularly robust
Chris