I concur with Doug- an active x-over is the way to go. You'll eliminate the low frequencies being fed to the Spicas which should, in turn, help to eliminate the thump you're getting. (The thump is probably the speaker trying to play some lower octave input which then impacts the remaining material the speaker is trying to play.) You'll also be able to play much louder if you have a powered sub (or separate amp for the sub) with an active x-over as your existing amp is freed up from power-robbing low frequencies and the Spicas don't have to attempt to play the low stuff, which leaves them free to do what they do best.
If the Spicas really work for you, then experiment with a sub. If you are open to new speakers, it might be a little less expensive in the long run to make a change.
Note that, in my limited personal experience in toying with subs, I've found that one shouldn't expect a sub to play up higher than 60 hz.