I understand that it's difficult to build a quiet all-valve MC phono preamp.
Correct, unless used with a suitable input transformer. Low noise requirements for valves are all wrong for feeding from a low impedance source. Novelty value only to my mind.
Not often seen are phono input stages using a high-gm valve. John Fuller illustrated the advantage of that in his well-known phono amp of some decades ago. (In valves noise is inversely related to gm, with some relation also to input source impedance.)
One of the lowest noise input stages available is a well-designed triode cascode topology.
I prefer to design a phono stage using a feedback RIAA topology plus inherent sharp low cut-off filter. It follows easily from already present components, and provides useful (sometimes mandatory) filtering of sub-audio articfacts.
As Geco suggested low noise factor requires a low-noise power supply, particularly the input stage. Good dynamic range comes from using a high h.t.; I am not shy to start with 400V which leaves some room for good supply rail filtering.
And don't forget some mains filtering! It is easy and every bit helps.