I decided to switch to (Xfce 4.10's) Terminal
256 colors out of the box: CHECK
Works with Source Code Pro: CHECK
Displays every character in Quickbrown.txt (out of the box, on Slackware 14.0) while the font is set to Source Code Pro: CHECK (XTerm can't do this).
Kerns fonts properly: CHECK (rxvt-unicode messes this up)
Works with the Solarized color scheme: CHECK
Thanks to all who recommended it.