I A/B'd the Boston SPZ60's against the Polk SR6500 and found a big difference, IMO. I am a musician and a sound engineer, but keep in mind that audio is in the ear of the listener, so to speak. To my ears, the Polks were the finest sub $1k set of 6.5" separates I've heard to date. The sound was very pleasing and full, and not at all artificial. The Bostons' treble was too harsh, and even when cut back in the crossovers they didn't sound as natural as the Polks. Also, the Polk's mids hit harder and gave a greater range of sound than the Bostons.
Note: The speakers were installed in a Land Rover Discovery II doors, driven by a JL Audio 300/2 amplifier. Test music used was rock and pop.