My 15's had a cap value of 4.7uF. I have a later version of 15's than yours. I have no idea why yours is 12uF and mines 4uF's, the dudes at celestion must have had good reasons. I replaced the old knackered 4.7uf caps on the XO with the Jantzen's and loved the sound. I then wanted to try the Duelund bypass caps, I bought a pair and then bypassed the Jantzens, I can see a lot of members rubbing their heads at this point. Well, it worked very very well indeed and I have no intentions of doing any further modding to the 15's XO. The bypass cap has a very very low value so that all the original XO cap values are maintained and the voicing is kept in place. So I didn't work out the optimal value. All bypass caps have similar low values. Hope this roughly explains things.