Size Does Matter with pickups.. and the coils of Quiet Coils, Mini Humbuckers & Filtertrons are too darn small.
Cramming wire into small spaces inevitable results in a excessive Parasitic Coil Capacitance In these pickups Parasitic Coil Capacitance acts like an eq circuit and singles out certain harmonic frequencies most audibly on the low wound strings robbing them of important midrange frequencies which impacts severely on definition (piano-midtone) and attack.
The Gretsch Filtertron from some accounts has a twangy sound but when I played one recently I immediately heard the low E and A string sounded muffled and very indistinct. Mojotone Quiet Coil has the same problem. Only the middle strings had any kind of focus and twang, and even then it wasn't quality twang but more the product of upper midrange spikes that are the precursors to ear piercing harshness. I wondered if the pickups I tested were somehow faulty but on every sound clip on the Internet I heard the same thing, and then I read on some forums players complaining about the very same thing.
Players complain about the bad sound of the low wounds of humbuckers but Mini-buckers and Filtertrons are far worse simply because their coils are too small.
Compact (economy) pickups such as dimarzio Area (Strat) series suffer from the same problem.
Kinman refuses to make these types of pickups because we know there is no way to improve their sound because of the limited dimensions, in other words it's against our religion to use flawed designs.UPDATE 13-Nov-2019: Boss Chris is a tireless innovator always looking for solutions to problems. Kinman is very pleased to announce that we have found a solution to improve the sound of Filtertrons and Mini-humbuckers outta sight. We expect a sound with greater clarity much like a single coil and a fair degree of twang from the low wound strings. We hope to bring out a mini-humbucker late 2019 early 2020. The Filtertron fix turned out to be financially unviable and we have all but abandoned that project.