Those diminutive external boxes turn out to have an enormous impact on the sound!
People had been tipping me for some time that there were huge benefits in changing components in the external passive filterboxes, or by swapping them out entirely, to make room for bigger components. While I never had any doubt that this was true, I always feared for trading one virtue for another. You know: getting bigger bass but less treble air, or more dynamic but an overall drier sound. When a reader of this site suggested to me that I just try the filters that he had lying in the closet for some time, to make up my own mind, I jumped at the chance.
Original filters, fitted with Cardas connectors on the inlet side. This was neccesary because the Transparent cable's spades wouldn't fit the Magnepan's original banana-like connectors.
Current, upgraded filters, again, on the input side fitted with Cardas connectors.
Original filter on the inside, the only modification being Cardas connectors.
Upgraded filters: all components are oversized and needed a new enclosure. They were made or modified by Koos Schenk of TAF luidsprekers. That's all I know of them. But the proof of the pudding, as they say, is in the eating, and they sure sound mighty fine.
High quality Mundorf and McCap capacitors and Mundorf coils. Wiring is litze instead of solid metal.
The original filter really looks diminutive compared to the new ones...
So, the replacement filter boxes arrived and I did some comparisons in the presence of the owner. What was immediately noticeable was an increase in spaciousness, both sideways as in the depth plane. Also the midrange was much more natural, more developed and richer. Treble was smoother as well. What I didn't like as much was the bass fatness and an apparent lack of speed and attack. But the owner was happy to let me test drive the filters for a few more weeks before I had to decide. That was very fortunate. Even when at first I had doubts and felt reinforced in the idea that you always pay a price in some department as a trade off for improvements in other departments, eventually I got it all right. For sure the filters had to do some running in because they had been sitting idle in a closet for many years. But it wasn't only that: I had to do some major tweaking to the system in order to get my desired bass-tightness and dynamic differentiation back. What was certain during this difficult time was that whenever I placed the old filters back, which was easy because I resisted the urge to take out the expensive Cardas connectors and just purchased a second set), everything became worse. Sure, it was more upbeat, but the sound was comparably harsh and threadbare. In the end I don't even know anymore what I adjusted exactly, but it involved system-wide changes: extensionblock, powercables, rack adjustments, ceraballs instead of rubber feet, speaker toe-in, tilt and distance from the rear wall etc etc.
Because Magnepan speakers make it so easy to experiment with the external filters, I would recommend that you do. You can always swap back. However, there are two caveats. One is that it is much more difficult to build your own filters than you might think. Getting the roll off points right is one thing, but the end result is dependent on much more than technical data. I know this from a friend who designs speakers and crossovers. You can spend months and months swapping out coils, capacitors and resistors before you get precisely the sound that you want. This is why I have resisted modding my existing original filters, or to make my own. Second: be prepared to make system-wide modifications, because invariably, the new filters will have properties that don't fit right in with the rest of the system. Think of the new filters as new speakers, and you will understand that you can't just pop them in and expect great sound.