Penta I use speakerlink
Penta II can use speakerlink OR Mk2 Powerlink
Why complicate things with an adapter????
The BC9500 has both speakerlink and powerlink outputs so you can choose either.
I suggest that you go for Penta III. Earlier models will almost certainly have rotten mid range drivers. Having said that, the midranges could be rotten in III's as well so just factor in an additional £250 + to replace them.