I have similar speakers in my HT: Polk RTi8(front)/CSi5(center)/RTi6(surround)/FXi5(back) and SVS PCU(sub) which I'm enjoying immensely.

My HT used to be LSi15/LSiC/LSi9/LSi9 but these were relegated (!) to a multichannel, music-only system because I found the LSi series lacking in the punch and even (gasp) brightness I wanted for movies (and this coming from a tube guy).

I would strongly advise against mixing the LSi and RTi series as they sound noticeably different.

My favorite speaker of all the Polks is the CSi5. For HT, I think it is superior to the LSiC.

I think one option you have is keeping the RTi8s and adding a good sub.