Nagra VPA 845 vacuum tube suggestions

Recently acquired a pair of these monoblocks. They are older series, balanced input only. Interested in suggestions regarding the 845 tubes as mine need replacement. Manual says they need to withstand up to 1200V with 100 watt dissipation. Current tubes have the following label: M.N. 845 S.Q. I believe they are Shuguang. I have heard good things about the Psvane 845 but the tube has wider glass and would not be able to fit the metal tube cage over it.
Could always order from Nagra but before I do would like to know if there are better/different tubes available.