I run a set of RTi12's off my Adcom, essentially the same speakers in a different enclosure than the RTi A9. It's on a shelf on an open front/closed back entertainment center "credenza", plus the 5500 runs hotter due to it's MOSFET output transistors vs the bipolar output transistors on the 555. The 555 can be bridged to a single channel, the 5500 can't The 555 has a damping factor of 150-200 while the 5500 has a 700 damping factor. The damping factor is how well the amp can control the speakers, especially the woofers. As a result some have the opinion the 5500 has "tighter bass".
Essentially they sound slightly different and one can be bridged and the other can't, and one runs hotter. Both sound good, and it's a matter of personal preference which one you like better.
I didn't know any of this until after I bought it though, I bought it on a late night whim.
I'm either going to pull it out of the credenza and put it in open air, or set up a couple of quiet PC fans on the heat sinks to be safe. It's not hit thermal protection yet but electronics tend to live longer when cool.