Well my direct comparison between the REF 3, SP-11 retubed and SP 16 and other AR preamps were very revealing. I did not play with tube rolling or change cables, etc. I simply listened to music with one unit, unplugged it and connected the other and listened to the same music. The SP-11 (retubed mind you, with AR supplied tubes) was wayyyyy better than the the SP-16. It was not even close. I would even classify the Sp-16 as being a lower high-end or upper mid fi pre-amp. I'm not kidding. It wasn't close. However, the SP-11 was close to the REF 3 to the point where I seriously questioned buying the REF 3. The REF 3 was better, don't get me wrong, but the price difference, wellll, was it worth doing it. In my opinion, it was! I find myself no wanting to stop listening. However, I sold my SP-11 for around $2,300 and that was a fair price. So, whom ever is listing an SP-11 for $5,000 or even over $3,000 is asking way too much.
I have not heard a direct comparison between the SP-11 and the SP-15, but people who's opinion I respect tell me the SP-15 is on the same level as the SP-11 MK II.
enjoy