My experience was with a Yaquin CD-SD3 tube buffer with Jan Phillips WGTA tubes. In my system, it did make a difference, although I can't say that it was all for the better.
I originally got it for the reasons that you stated. To take the edge off of the highs in my solid state system. I was running an NAD C162 pre amp, Aragon 2004 amp and Paradigm Studio 10 speakers. The buffer did take the edge off of the highs, but it also made them sound rolled off. The bass got less tight and, sometimes, downright sloppy.
After getting the buffer, I got into a pair of Silverline Minuet speakers. No more edginess or glare, so I sold the tube buffer.
Many have referred to a tube buffer as being a bandaid. I think that in your case, this could be true, in the sense that you want to use one to "fix" a problem and is likely not the best way to go about it.
What's your setup, anyway? I have recently altered my system to consist of a Marsh P2000 pre, Aragon 2004 amp, Emotiva XDA DAC and the Silverline Minuet Supreme Pluses and I don't get even a hint of glare or edginess.