every speaker builder has their own signature sound, so the odds are slim that you will fund a similar sounding speaker to the XS-650. Since they all sound different, its up to you as the consumer to decide which speaker actually sounds best to your ears. The XS-650 is an awesome speaker, and the HT-6 is a perfect amp to drive them. If you are able to demo the XS-650, Im sure you will be impressed with it. If budget is a concern, then I suggest the XS-65i and the HT-6 amp. The new 65i is a huge improvement over the original xs-65. I have had a chance to demo them side by side recently, on a couple of different amps.
The SD-6 is a ton of power for the XS-650. We did 3 pair of XS-650 on an SD-4 and was blown away with the output. The R/F M626 is a good speaker, but the mid-bass from the XS-650 is outstanding, and the overall depth and clarity is phenomenal. At the end of the day, getting a demo is important, if possible.
For a Rev 3-Some, go with the an SD-6