What are the most under-appreciated products?

OHM ACOUSTICS In 1988 The Audiophile-File placed the 4-XO in the ranks of Martin Logan Monolith, Apogee Duetta II and Snell Type A. Here what the MK-2 series sound like now.

We should never forget the late Lirpa Labs...inventors of such great audiophile delights as:
Steam-powered turntable
Compact Disk Washer
Stationary record player...the disk was stationary and the Stylus ran around the record
And many other great items untill the demise of Audio Magazine...

Cambridge soundworks speakers - as good as Bose for a lot less. Stereo Review said they are the best speaker value in America and the price has dropped since.

Have a listen - for the money, it is a great deal for a small space somewhere.

whipped cream

I think Clements Audio speakers are great for the money but seem less known than they ought to be. To a greater extent, however, would be JJ Electronic tube amps. My 243 preamp is work of art...stunning sound...has a very nice tube phono amp...compares favorably with some very expensive and well respected company...and damned affordable at around $1600, to top it off. Very little known in the US...but just damned good!