It is a waste of money to
biamp, not to use an external amp
No receiver will have the sort of power a decent external amp will give you. The LSi7s are 4 Ohm, but they are also far easier to drive. I owned them and drove them off a receiver for a year so you can get away with the 7s on a receiver. The 15s would be better off with an amp if you want to turn things up. The risk is damaging the drivers not your receiver. If you don't have enough power and crank it up, the tweeter usually fries first, and the LSis are fairly well known for this which is why an amp is usually recommended for them. In terms of an amp, you can look at a 5ch amp like the Emotiva XPA-5 which should be plenty for a full set of LSis. You might also look at something like the XPA-3 to run just the front three speakers as you will likely want an amp for the matching LSiC center too, which is also harder to drive than the 7s.
The guy that sold you the speakers was clearly not knowledgeable - don't ask his advice again.