Most tube buffers will help with impedance mismatches. They are buffers. As Almarg was saying a buffer stage may help. Most seem to think their purpose is to add tube flavor. I don't think they do, my view is that a tube system excels at imaging.
Impedance matching device
I did some searching. There is no shortage of threads on impedance but I don’t find the answer to the following question.
Assuming you have an impedance mismatch between your pre and power amp is there a device which will correct or compensate for this? Can someone supply a link if this beast exists and doesn’t degrade the signal?
I just tried a new used amp (Sumo Andromida II) in my system and I immediately noticed how much more effective the volume control on my VTL 2.5 pre is than it was with my currently out for repair Aragon 8008ST. By the specs the VTL/Aragon should work but it is on the outer limit of what most consider acceptable impedance matching. VTL output impedance is 200 ohms. Aragon input impedance is about 23 K ohms.
It does work and I’ve lived with it for years but now that I’ve tried something else I am wondering if perhaps the performance of the Aragon was being affected by the borderline mismatch. I’m suspicious because I found the Sumo to sound as good if not better than the Aragon in my system. I expected the opposite. I do realize there are many factors involved but this is a starting point.