I own the 6.5s. They require a LOT of break-in time. I kept a log for the first 600 hours of use. For the first 100 hours they sounded boomy, very dark, and with restricted dynamics. After 200 hours most of the boom had gone away and they started to open up in the treble. At 400 hours they had become fairly neutral and I really started to enjoy them. They changed very little between 400 and 600 hours, and I was pretty sick of keeping the log, so I quit counting. Even so, they have continued to get more dynamic and the treble is really sweet now. I'm guessing I have around 800 hours on them.
I have tried several amps with them, and while the midrange of tubes was very nice (Cary V12 and 805C), these speakers require the control of a beefy solid-state amp to tighten up the bass. My solution was to buy the sweetest sounding solid-state amp I could find (Rowland Model 10). This is a very nice match. Stay away from bright solid-state amps (Aragon, Krell, Bryston, etc.) The tweeters on the 6.5s will not tolerate a bright amp.
After break-in these speakers are basically neutral, favoring the warm side slightly. This makes for fatigue-free listening. Also, you can play them very loud and they never strain.
The 6.5s are very musical. I am a former trombone player and am very critical of how a speaker reproduces brass instruments. The 6.5s do brass better than any other speaker I auditioned. I listened to several speakers from $4K up to $23K. The 6.5s were my favorites, regardless of price. They also do a great job with other acoustic instruments. I listen to jazz and classical mainly with some rock and roll and bluegrass thrown in. The 6.5s do a great job with all of it.
The extended bass of these speakers is very addictive. Very few speakers get to the low 20's like these do. When I was auditioning the Rowland, the dealer was using Avalon Eidelons. After being used to the 6.5s, I couldn't believe how much information the Avalons were missing. I kept thinking to myself "these Avalons have no low bass response at all!".
As for negatives, you have to be careful with speaker cable and interconnect selection. You can easily make these speakers sound too boomy with the wrong cable. The extended bass response magnifies a boomy-sounding cable like a Monster. Also, you can suck the life out of them with anything with a treble roll-off like Audioquest. The best approach is to buy neutral sounding cables. I'm using Kimber Bifocal-XL speaker cables and Cardas Neutral Reference interconnects.
Room placement is also a challenge. My speakers sound best when placed 5 feet from the wall behind the speakers and at least 3 feet from the side walls. You also need a room with fairly large volume or the bass will be boomy.
Expect a discount when buying. List is $6399, I paid $5200.
I haven't heard a perfect speaker yet, and this one is no exception. However, I don't know of any others even close to its price that offer true full-range performance and are as listenable as the 6.5s.