Everything has been said about these drivers, and by people who are much more knowledgeable about them than myself. Several articles are present on the net as well as on the magazines. However, quite a lot of time has passed from the more recent articles; so I think that a "refresh" should be not so bad.

The history (just a little bit).
For those of you that are in hi-fi from the last years of the "old" Seventies, the name Lowther was almost unknown until the "low power/high efficiency" systems have come back into fashion. The purist and minimalist philosophy, which considers the single ended triode as the "king of amplifier" finds its natural match in the wideband speaker. If we add to this that often, for physical reason, the wideband drivers are normally very sensitive also, we are game. Lowther dealers are very rare; fortunately, thanks to several Lowther clubs worldwide, to the enthusiast "Lowther guys" and to a few intrepid dealers, the Lowther sound has been brought to many houses, often radically changing the way in which many of us listen to music.

The drivers
Until a little while ago, Lowther production was centered on just two series of drivers, the "A" and "C" series. "C" drivers use a light paper diaphragm with concentric ribs. In the center a "Phasing Stabilizer" (the famous "nose") is to be found. Magnets are all FEROBA type. In "A" drivers the cone is made of thicker paper and the ribs are radial. These drivers use the phasing stabilizer also. Magnets are made of AlNiCo. It is generally recognized that "A" series drivers sound better than "C"s. This is to be attributed mainly to the magnet (AlNiCo for "A" drivers, as said above) but the different paper used for the diaphragm is also important. However, I have listened for a long time to a PM6CS (my first Lowther) and I have been enjoying it very much.

Since a few time other two driver's series have been released; Dx and Ex series. Dx serie uses rare earth magnet that are stronger and lighter respect to the AlNico and Feroba types. Lowther claims that this provide several advantages due to the avoiding of the sound reflections on the magnet (typical of Feroba type) and to the reduction of mechanical stress on the basket. Also These drivers use the phasing stabilizer. ExSerie is a further evolution of Dx; an acoustic chamber is fitted in the rear of the driver and the phasing stabilizer is substituted from the phasing equalizer. The combined actions of both devices avoid the parasitic oscillation of the cone and then the "colorations" often reported from ??non amanti of these drivers. I have no news regarding sonical performances but seems that the heart of Lowther people remain with "A" series. Common features for all drivers: very light paper cone, air gap very narrow (just 1 mm), dual wind coil, very strong magnets; all of this in order to have a time response that start from "very good" for the cheapest models (PM6, PM7, Dx2, etc) to the "exceptional" for the big brothers (PM2, PM5, PM4, Ex4, etc).

Tweaks
Lowthers are probably the most tweaked drivers in the world; just to "mitigate" their temperament or to enhance their performance, these drivers are often modified by their owners. The first and simplest tweak consists in fitting a small quantity of wool fiber between the main and the whizzer cone in order to reduce the acoustic interaction between the two. A variation of this tweak is to use a foam strip. Only on "C" drivers, sometimes it is useful to cover the magnet with damping material in order to avoid soundwave reflections. Another very famous tweak consists in stiffening the cone by applying a lacquer like Dammar or the more famous C37 to it; this should allow better high frequency extension and lower distortion. I haven't tried any of these tweaks, the only suggestion which seems sound enough to me, mostly in consideration of the high cost of the drivers, is to perform easily reversible modifications, and only if you really feel the need.

The loudspeakers
Because of its features, it is not so easy to extract the best sound from a Lowther driver. As Ciro Marzio wrote in the CHF magazine "…Lowther drivers are designed to be fitted in a horn…"; this means BIG and, of course, very complex loudspeakers. Starting from the simpler models like Acousta, Medallion, Fidelio, Mauhorn, we get to the more complex and better performing Side Vivace, Opus One, Audiovector. I have listened to all of these loudspeakers (Opus One and Audiovector excepted) and I have to say that, while the "small" models provide reasonable performance, a really deep and, at the same time, strong bass came out from the big models only. The correct match between driver and enclosure is paramount. I have listened to my Vivace fitted with a PM6A and with a PM2A, and I have no doubt that a better balance was reached with this latter. Recently, front horns have been developed for Lowther drivers. I have listened to just one of these and, while the performances are very interesting, I have to say that this came at the price of a much higher complexity of the audio system. The front horn doesn't and cannot, for physical and space reasons, cover the whole spectrum (20-20000)and is limited to a cut-off frequency around 150-200Hz: this means that you need a multi-amplified system. This fact implies that a solution of this kind is strictly for "enthusiasts".

My two cents on their "personality"…
Living with a Lowther is not so easy. Its exceptional performance in the time domain poses high requirements to the whole audio system, beginning from software. There are several CDs (or LPs) that are simply "terrible" if played through the ruthless precision of a PM2. Any little component substitution in the system becomes macroscopically evident. Substantial differences are present day by day, probably due to electrical power quality or to air humidity variations. This also happens with any good audio system, but the Lowthers just ac like a magnifying glass.

Regarding amplifier matching: I would like to make it clear that Lowther drivers AREN'T FOR SINGLE ENDED TRIODE AMPLIFIERS ONLY. While you will probably obtain the best results with this kind of amplifiers, Lowther drivers sound good with solid state amplifiers (it just depends on which solid state amplifier, obviously). I think that it's more of a matter of taste.

However, it must be said that the kind of sound these drivers provide is really one of a kind, and it is just not for all tastes. Some people only hear and can't get through its negative side and deem it "colored", while other people hear "the other side" and judge it dynamic, lively, real.

A lot depends on the setup, often not optimized for this kind of drivers, which makes the results downright nasty. A "burn in period" for our ears is also always necessary in order to "tune" our senses with this peculiar sound.

However, it's true. You can either love or hate Lowther drivers.

And I love them.

Bibliography:
Ciro Marzio, Costruire HiFi n.28
Haden Boardman, Sound Practices
Joe roberts, Sound Practices
Marc Wauters, Sound practices