I just want to make two more points, because everything else has already been covered.
First, adding a powered sub will actually help out a lot. Not only will it not take power from the receiver, it will actually reduce the load on the receiver. Right now you're running the RTI8's "full range" (all audio goes through them). Bass notes take LOTS of power in comparison to the other frequencies. So moving those lower frequencies to the sub, helps the receiver as it doesn't have to use power to produce those low notes anymore. You'd obviously mostly notice this when playing bass heavy audio.
Second, the wattage ratings on speakers doesn't really tell you much, and not all watts are created equal. Some amplifiers have much higher current behind their watts, and they do in fact drive most speakers better. Most speakers are damaged or "blown" through amplifier distortion. Amp distortion kicks in when its trying to produce too much wattage. So if you're turning up your receiver a lot and getting distortion, its time for an amp (or something.. like a new receiver, lower listening levels, etc) otherwise you could end up blowing your nice "new" speakers.
Case in point, I have a 75wpc denon (older model), and my neighbor had a 120wpc sony. We tried out our receivers side by side, and there was no comparison. We tried them on his system, and his first words were "I've never heard my speakers sound like this". He went out the next week to replace the sony. I'm not saying that I have something amazing, but am just pointing out that not all receivers of the same wattage are the same (just like PSU's).
Also, I just got a pair of RTI a5's (similar to the 8's), and they're quite good, so enjoy them as they're totally an upgrade from the R150's.