Gain stage and output buffer in Tube DACs

Please help me understand what the gain stage in a tube dac does. (for eg. the Jolida Glass FX DAC)

I understand  the gain stage helps control volume level and in in such DACs it is implemented using tubes. But I am not clear if it is attenuating or amplifying (voltage/current/both) the signal ?

Also, wondering about the related term "output buffer". From what i gather is merely something that helps match impedance... but not sure if it has anything to do with "volume control".