If you mean Boston Acoustics A100 speakers, they have a fairly high sensitivity rating (90dB) so don't need crazy amounts of power to drive them to a fairly loud level with minimal distortion. That said, I think you might want to set your sights on an amplifier/receiver with a bit more power than those you mentioned. I would recommend something with 40-50 watts per channel or more...unless you don't plan on cranking it up at least once in a while.
Power is not just for loudness...you need enough power to keep from pushing the amp to the edge of its capabilities (or beyond!), because that's when amplifier distortion starts to become a real, and quite audible, issue.
As for differences in sound...speakers will certainly play more of a role than the amp/receiver. But, there are certainly differences in the sound of one amp vs another...driven primarily by differences in design, rather than brand. In other words, similar designs from different brands will sound more alike than different designs from the same brand.
I am not very familiar with Radio Shack/Realistic receivers, but can tell you with certainty that across the Sansui product lines and over the years that they were made, there is a fairly broad range in their sound "signatures". The XX1 series are quite popular, but perhaps not as "accurate" as some of their later series.
While properly matching them from a technical standpoint is important, finding an amp and speaker that just sound good together - synergy! - is just as important as the specs. I would bet that those BA speakers will pair quite well with a Sansui xx1 receiver....even the 551, if - as noted above - you will be listening at more moderate volumes.
Good luck finding what you need...enjoy the hunt.
