Some of the HTIB sets from Yamaha (and Onkyo) used decent AVRs. What model is yours?
Usually for a sound upgrade you'd want better speakers and sub primarily (as that's where most of it is derived).
What do you want in the way of a new avr feature-wise that your current one/use lacks?
As far as a 2ch rating, that's just to make the numbers for the sales/marketing a bit bigger. OTOH an ACD (all channels driven) rating isn't particularly useful or relevant to usual surround content either (if you like an all-channel stereo mode that could be a bit different). In most cases each channel has the same capabilities at any one time. The speaker wattage ratings are more about a limit of continuous program, not something particularly useful beyond that. Your speakers are on the low side sensitivity-wise so need a bit more amp than some others might. Takes a doubling of power to gain 3dB....so a speaker with 82 dB sensitivity would need twice as much power to match a speaker with 85dB sensitivity (generally, it's not an exact thing, more puts you in the neighborhood as other factors such as impedance swings and phase angles of your speakers at particular frequencies....your speakers' 6 ohm rating is nominal, it likely spends time near 4 ohm as well as above 6 ohm....)
You can use an amp rated less or more than the speaker, it's the actual use of the volume control that determines how much power the speakers will require at a given time, a lower power amp may limit volume, tho.
Use this calculator to get an idea of the relationship of speaker sensitivity and distance from the speaker and spl....
http://myhometheater.homestead.com/splcalculator.html
