I own the Scosche EFX RLC, PAC LC-1, CT Sounds Universal Gain Knob/ Voltage Meter, and the Axxess AALC.
The Scosche EFX RLC is by far the best of the bunch. It has lasted the longest, has a non-visual notch you can feel while turning that indicates the half-way mark, and it is the most linear of all the ones listed. Save yourself some hassle, buy it.
I ordered this to compare with others I had, so I could post a comprehensive review on Facebook. (google Universal Gain Adjusters under "Bassheads Unite" page). Function and price were great. It had no distortion and was the cheapest of the 3. So far so good... However, a couple things utterly ruin this product.
1) It may have been the cheapest, but you can definitely tell it is the cheapest. It feels just cheap and flimsy. I actually broke the mount that comes with it before I could even install it and had to super glue it.
2) It was the only one that didn't have a removable knob. I like to flush mount controllers, and to do this I have to be able to remove the dial and put it back on after I mount it. The dial feels awkward and much more uncomfortable than the others.
3) THIS IS WHAT MAKES THIS PRODUCT UNUSABLE AND IN MY CASE, POTENTIALLY DANGEROUS!!!!! It is WAAAY too easy to turn the dial. I have a 3 year old son that rides in the backseat daily. Several times I unknowingly turned the gain full tilt by barely brushing it with a drink or my leg. I have NEVER had this happen, even once, with the PAC LC-1, Scosche EFX RLC, or even the CT Sounds gain controller. It happened 3 times in 2 days with this one!! Thankfully, my son wasn't in the car during these "Bass Scares". My hearing is mine to damage, but who am I to damage my son's? Very careless!!!
I am highly disappointed in this product from NVX. I am a huge fan of theirs btw. I have the X-Series Tweets and 6.5" Door speakers and I absolutely LOVE THEM. I expected much more from this product.... I would redesign this product asap. It is stupid to have a cheap bass knob tarnish an amazing company image.