I have owned the 8W7 and the DD1508's...I am currently running 3-SS RL-i8's in a ported enclosure with 2500RMS on them in my daily vehicle. The Soundsplinter Orphan 8's are a great option, but to be honest the 8W7 will edge them out unless you plan to run a pair in a similar configuration that you have now.
The 8W7 is a hard sub to beat and if it is begining to sound like you are taking it to its limits I would suggest having the sub looked at or replacing it with a similar sub. I do favor my RL-i8's, but mostly because I should have kept the 8W7's I had slightly before them. I chose to get rid of those to experiment with the RL-i8's. I love my Soundsplinters off the same power though.
There is no need to change your whole setup, just stick to what works.