I've tried both Labella Deep Talkin quarters and tapes on my fretlsess basses. the tapes seem to have more voice and character than the D.T.'s. Also trying out some Roto Monel flats on the fretless 6, (E-F) not really slap-able but tapping works....could just be technique.
.02 here. I have been using TI flats on #196 and they mwahh just wonderfully. I believe the setup is more important (flame the man right here) than the strings and I have several fretless basses over the years.

