Here are just a few sites that I think could help us get more members though server lists that use a voting system to simple forums.
-http://minecraft-server-list.com/sort/Tekkit/ (used for all Minecraft servers but has a Tekkit search option)
-http://www.planetminecraft.com/forums/tekkit-servers-t95671-180.html (may help but there are better options as always )
-http://forums.technicpack.net/forums/open-servers.28/ (A simple detailed forum post could bring new members if kept up to date)
-http://www.reddit.com/r/tekkitservers (not quite sure how this one works. I'm not accustomed to Reddit)
-YouTube videos showcasing our amazing server. A lot of people overlook this. Even if someone is really ambitious they could do a Survival Let's Play that could bring in some new members.
These are just a few I found doing a quick Google search. There are other sites and I will get in touch with the people who helped me with advertising on my server to see if they have any additional ideas and/or suggestions.