Im suprized youre getting better growth with the 15k's opposed to the 10k's. All the reports Ive read, (Sanjay/JBNY and others) show the XM 15K to be averaging one third the output par of the 10k's of equal wattage. There have been reports of folks loving the color but switched back to 10k's plus actinics because growth was dramaticly reduced. Is it possible the old 10k's were so worn out the new 15k's were actually brighter and bumped up the growth? I just dont see how you can get better growth from a bulb with one third the output par, but thats just me....