Slingshot, One of the characteristics of the Mercury which makes it so nonfatiguing and natural is the lack of edge and any spectacular "electronic sound". It does require some break-in and after break-in sounds best after about an hour warm up. You, probably know all this. I suspect the customer who who sent it back had a component mismatch or was looking for a more upfront sound. I am sure it is more complicated then that. AC line conditioning, isolation, "wires" all make a difference. I must admit, I have never heard lifeless or dull sound coming through the Mercury. I understand however the Mercury 2 improvements were Kara's attempt to add a little more life to the sound, but told me in an email not to upgrade since I was so happy. I previously used the Pass 2.5 which is an excellent very neutral preamp and highly recommended. The Mercury however goes beyond in harmonic accuracy and timbre. I find the images more true to life with more bloom. This is a fun hobby. I am glad my system however is at a point I can just sit back and listen, thinking more about the performance and adding music to my collection.
