psychotropic - unfortunately I have not heard the Usher S520. Barring that, I would pick the PSB Alpha B1 as the best speaker among the other choices that you have proposed.
There could be one or two speakers that may be able to trump the Alphas in one or two areas, but overall, the Alphas present what I like to think of as a coherent package. Lacking nothing. Especially when you consider the price. And the human voice is a delight to listen to on the PSB Alpha B1. If Indian music is anywhere on your radar, this should be high up on your list of considerations (performance with human voice, I mean).
I find the Diamond 9.1's tweeter to be too saccharine sweet. Especially with soft female voices. I dont find it to be natural. But I can see how it can be preferred by people.
The MA BR2 is probably the closest competitor to the Alphas. It has better bass capability but is however a somewhat reserved speaker inherently.
The Polks, I am afraid, are not very musical to start with. Not at this price anyhow. Their Lsi series is outstanding.