I reviewed the original Monitor Audio Silver series a few years ago and found them to be a bit lacking. When I pulled them apart, I found an economical OEM tweeter, no bracing and low grade electrolytic crossovers. It was then I realized the sum of the parts was responsible for the boxy, thin compressed sound I was hearing. I cannot directly comment on the new Silver Si series, but after my review, Monitor Audio assured me that they upgraded their crossovers with better parts and have since then made other upgrades to their Silver Series line. However, I have reviewed their Gold series but much enthusiasm, but they are significantly higher in price.
Between the Axiom and Aperion, it depends on what you are after. I personally feel the Axiom has the edge for midrance clarity and focus, but can be a bit hot on top, while the Aperions have a more pronounced bass section, laid back sound. Both are good values. The other positive with the Aperions is their beautiful cabinetry and finish yielding a high Wife Acceptance Factor (WAF).