When i was searching for speakers, the polk db651 got excellent reviews. I found an 's' version and a regular version -- i had no idea the difference, but found some information about it on this forum:
To call out specifically, aside from the power input specification, there are these significant differences...
1. the 's' version is shallower mount on the FRONT. The non 's' version uses a swivel mount for the tweeter, which protrudes a relatively large amount out the front of the speaker.
2. The surround on the non 's' version is much wider, and i am willing to bet (from listening) that the speaker is a lot better at making low bass.
3. the baskets are different -- while the spokes and magnet mount appear to be the same, the mounting trim is different-- seemingly thicker on 651s interestingly enough
Sound wise the db651 and db651s differ significantly. (i now am using aura 6200RPM all the way around instead of these polks) Before i thought the sound difference could possibly be the difference in front and rear door mounting, but it was not the case as the aura's sound virtually identical in the front and rear.
The db651 has significantly lower extension as well as significantly less low midrange than the db651s.
Both the db651 and db651s were more bright than i would like -- i was running -6 to -8db on all frequencies above about 7khz. However, the Aura RPMs are bright too- so perhaps it is something to do with the full range class D amp i'm running? anyway...
Here are some comparison pictures to help show the differences