The LSi15 is essentially a LSi9 with some extra woofers. Personally, I felt the mids were what where lacking on the LSi line, especially for the price.
What size is your room? How do you have them placed? I kind of already said this in your other thread, but if you want great bass, you aren't going to have an easy time getting that from most mains even with a big amp. In a large or square room? That's even harder.
When you say bass is good on the 82s, I'd still have to say I can't imagine they are going to compare to even a good $500 sub.