Hi, I have a pair of Teesdale SP2 speakers which are similar except they are slightly smaller & have just one, slightly larger, bass unit.
Both the white midrange unit & the isodynamic tweeter were patented at the time. The white midrange unit with spiral holes was developed using laser holography & was known for having very low 'electrostatic levels' of colouration & distortion within its intended frequency range.
These units were used in a number of Rank Wharfedale/Leak speakers over the years until Rank closed down in ~1980 & sold the Wharfedale brand name.
I also have a pair of massive Leak 3090 speakers (from 1979), these use the ISO tweeter plus a version of the white midrange unit with a larger magnet + lower mid + 380mm bass in a separate box !
These were the last in a long line of Rank Wharfedale/Leak speakers ending in the late '70s early '80s. & I believe that they were & still are underrated. Both the Dovedale SP2 & the Teesdale SP2 are extremely heavy due to their large magnet assemblies + substantially built cabinets.
The downside is that after 40 years the bass driver foam surrounds will probably have perished. (My Teesdales have been repaired, it is much better to get an expert to do this than to replace with an alternative, unpredictable unit).
Also the crossover capacitors will have degraded; as I mentioned, these speakers are capable of very low levels of distortion & colouration - if you can hear any high frequency distortion, it is likely that the capacitors have increased in value & are allowing 'out of range' lower frequencies through to both the tweeter & midrange drivers. At worst this could blow the now very rare ISO tweeters.
I replaced the capacitors in my Leak 3090 spkrs with some fairly expensive polypropylene ones, the change was revelatory. They now sound great with a high end hi fi system with much improved clarity & low distortion, both at very low as well as high sound levels. This means that they are capable of reproducing the wide dynamic range from hi res sources to great effect...
Replacing the caps is easy if you can use a soldering iron, replacing the bass driver foam is best left to a specialist.
Properly restored these models can sound very good in a modern high quality sound system as they are capable of very low levels of distortion/colouration. I have found that all these models benefit from being driven from amplifiers with big power supplies (not necessarily high power) & high damping factors.
Whilst the resale value of the speakers overall is not great, people will nevertheless pay high prices for the rare ISO tweeters...provided they are working....
Enjoy.
Andy
