What are the most under-appreciated products?

Mod Squad “Soft Shoes.” This inexpensive footer is my most used tweak over the last twenty years. I currently have at least 25 of them in my system.

For hardware, the Sound Valves Stereo 70 amp. This late 1980’s modernized version of the Dynaco has provided good sound and no repairs for more than 14 years. Two pair of these bi-amp my B&W side channel speakers for home theatre.

1. Foundation Research LC-1 and LC-2 dedicated passive in-line power conditioners. Foundation Research is a small mfg'er in Ontario, CA.

Marty DeWulf of Bound for Sound recently reviewed the LC's as perhaps the best line conditioners available, sold his old line conditioners, and now uses these in his Big Rig.

2. McCormack DNA-2 Revision A amp. The revision A means it has been rebuilt by Steve McCormack with some of the best aftermarket parts available. An excellent amp using single-ended connections. An awesome amp using the XLR connections via the 2 additional volts.

3. AudioPoints from Star Sound. Installed them under my components, on my rack, and under my speakers. Could not believe the improvement after the mechanical break-in period.

-IMO

The Taylo reference monitors. Nothing touches them - at any price.

Thor amps and preamps.

Interesting so far, but I've heard of all of the above :)
C'mon, somebody stump us!