Despite the fact that this is an 80+ lb beast with casters on the back (!), the back panel drawing and the owners manual state the power consumption as 800W (6.9A@115VAC). Points off for the Rotel WebSite, since their spec sheet says 1200W. Two out of three, I vote for 800W...
There are way too many unknowns in the post to be able to comment correctly. If the amp comes with a 20A IEC (highly doubtful), and a 20A cord for the US market, you wouldn't be able to plug it into a 15A receptacle, due to NEMA plug/ receptacle configurations. There isn't any information on the age of the house, and what edition of the NEC it was built to, if it was at all.
The owner's manual has the following note:
"The RB-1090 has a built-in surge attentuator to prevent opening the electrical circuit breaker or fuse at turn on." It sure smells like an amp problem, but I'd have the whole panelboard looked at first. As in, replace the whole thing, or at least have all circuit breakers replaced. By a *real electrician, of course...