finding a definitive answer on bass blockers and harman kardon speakers is hard. But I'll share what I've learned, for verification from the more knowledgeable types in the hope that it might help someone.
It seems that 80khz is a good point to cut off, otherwise you lose too much. This requires capacitors of the order of 470 micro farads, and they can be fitted inline on the positive wire of the speaker. This solution is used by hk themselves if you look at the back of the tweeter you will see the same thing, just using a 4.7 micro farad capacitor, because you want to filter different frequencies to protect the tweeter. The rationale here is to do the same to the midrange speaker, and choosing an appropriate value to filter the lower frequencies from the midrange speakers, because they can't handle it and just distort.
Using cheap electrolytic ones is a bad idea, the right choice is non polar ( you don't need to worry about polarity of the capacitor), ideally polypropylene.
Bass blockers are not a 'good' solution I'm told, and a true audiophile will probably shudder. This is about cost effective problem solving, and is a compromise I imagine, as they 'throw away' a lot of music and are not that effective I'm told. If Coolcat does find the crossovers he's offered me, I can compare and hopefully I will be able to report back what the best solution is for getting the best out of hk kit in 2015. The idea is simply to cut back on what others report as 'boominess' caused by the full range hitting the door mid range speakers that can't handle the lower frequencies. Then you can leave your sub to do the work, and you can happily crank up the volume a little more.
For reference, these are the type of capacitors you need:
http://www.ebay.co.uk/itm/331689707108