Eldartford, your logic is flawed. Even though it is true that components are rated at some value plus/minus some percentage, it does not follow that a different value will sound the same even though it is within the rated specification. For example, if you built a crossover with all caps that are + 20% and another with all caps that are minus 20%, they would have decidely different cutoff frequencies and they would sound different.
As to the original weaker bass, stronger bass question, this could have something to do with the capacitors in the amp. The dielectric does change over time as voltage is applied to it. This affects the capacitance as well as the effective series resistance. I have no idea what the topology of this amp is or what type of caps are used, but I would not rule this out. Sean has given sound scientific explanations for his response, Bomarc, I would be curious to hear the reasoning behind your "nope."