if you still want to use the windows volume control, don't use the cl-rlc. it requires that the external volume controller be plugged in, so it would be more of a pain then other line drivers..
i used to use a 3 channel version of the pheonix gold one you linked to-- it worked great-- until i blew it up...(it doesn't like +12v across the negative rca connections)
but it sounds like there is another issue at play here. i have a mrp-f300 for my mids/highs, and it barely gets warm... and i believe my gain is around 1/2..
check that the speaker resistance is correct(this amp is 2 ohm stable, so you you need at least that at every channel), and that no wires are shorted for starters..