I would suggest going to a gear oriented forum.
they will often do shoot-outs comparing diff pre's with audio examples.
most of these will be high-end stuff, but it should give an idea.
If your willing to make an investment check out "API lunchbox" Its basically a 6 space rack with a power supply. Many of these High end preamp manufactures are making modules of thier preamps that fit into form factor at greatly reduced prices.
There is a pretty good selection in the 500-600 dollar range.
I picked up a lunch box on ebay for 475, and I saving for a module now.