At MatrixHiFi you'll find various comparisons, some with graphs to illustrate certain key points of assorted equipment. Now the language isn't english, but if you've got the capability "right click" and translate, you should be able to make some sense out of it. For example, in a comparison of the Halcro DM10 preamp ($15,990) paired with the Mark Levinson No. 333 amplifier ($8,495) it was found that the users could not distinguish that combination from the Denon PMA 860 ($unknown) Now this doesn't mean that all the links on that page lead to a null result but you'll have to read to find out which products resulted in differences. Maybe the results will surprise some.