[Amps] question ref: matched final tubes

[Dan]

I have an Ameritron 811H using 4 811A tubes. seems much of my problems are behind me as the amp seems to work vry well now. Question I am the type that likes to have parts such as the tubes onhand just incase. From what I have read the 811A tube of fairly rugged and should last a while. question though.. I see 811a sold at RFPARTS as one two three or four. Does the 811A require a matched set when swapped out? or will buying a "spare" be ok..