Hey man I know this is delayed, but get em if you can. I have a pair of perfect condition all original S15-3's which are the stage version of those, and they sound great. No, you won't get bone pounding bass out of them but they are probably the brightest, warmest pro speaker around because of the cone mid range and the 3-way design.
And sentry's will have better bass then the FR300's or S15-3's at low volumes because of their intended use but the FR300's will perform better the louder you put them, believe me, they'll open up.