I just received the USB-XIII and used it to connect both a PB MSP-5 DAC and a Pacific Microsonics DAC to a Sound Galleries Monaco 2015/HQPlayer server in my system.
I haven't done detailed comparisons between the USB-X and USB-XIII, and I don't own or have listened to the Merlot, but I can tell you, without getting into audiophile descriptors, that the USB-XIII raises the MPS5 performance to another higher level.
And the same is true of the Pacific Microsonics DAC. How much of that is due to the USB-XIII, or the SGM or other improvements to my system, I can't say. But frankly, at this point, I'm enjoying the system too much to fiddle with the comparisons. But without question, the USB-XIII is a significant improvement over the USB-X.
Of course YMMV, but the USB-XIII is worth an audition if you have the USB-X.
However, I did speak to Andreas Koch (prior to purchasing the USB-XIII) about how the MPS-5 with the USB-XIII would compare with the Merlot. He remarked that their performance would be very close and the choice would come down to personal preference.
Which leads me to believe that the anecdotal comments about the Merlot being better than the MPS-5 were probably based on the MPS-5 using the USB-X rather than the USB-XIII with DSD4x capability.
Nevertheless, I also still look forward to comparisons of the Merlot and the MPS-5 with the USB-XIII.