I'm using my Pro's with a Yamamoto A-08s 45 SET amp currently. I have ordered Red Wine Audio Signature 70 monos also which will be here the week after next. I'm using an old Hafler 220 for the subs, to great effect.
The "brain" of my rig is a TacT 2.2XP. Playing around with it, I fully believe this combination (with a good DAC) would satisfy just about any audiophile on the planet.
There have been negative comments about the Definitions, though no direct problems I've heard from owners. Generally, these are attributed to "virgin ears" who don't "get" the non-XO'd sound. To some degree, I think this is correct. I had a fellow over who admitted after our session that it took him a half hour to adapt. Some people connect immediately, others never do. This is a primary concern in the "store audition", as well as a quick visit to an owner's home.
I've owned homebuilt single driver speakers, the Druids, the Definition 1.5's and Pro's. They're all excellent if you like their sound and get better up the ladder. The Definition 1.5's carry the tone of Druids even further while adding significant bass extension, though I was never able to get enough bass pressure out of them. Keep in mind I have rap, dance music, and metal in the rotation.
The plate amp in the 1.5's has been blamed for the difference between 1.5's and Pro's. I have no doubt this is a factor. For me, with the TacT, I set the crossover higher than the 1.5's, running the XO with shallow slopes at around 65 hz. This allows the mains to run lower and the subs to run higher, which permits incredible, unidentifiable blending between the main and sub arrays. With steeper slopes, bass presence was very strong, but with an easily identifiable handoff between the mains and subs.
Even with the Pro's I always thought there was a midrange-treble area that could get overstimulated with some music, maybe 1 khz-3 khz. Mostly, distorted guitar music would accentuate this area. I always thought it was an unaddressed byproduct of the wideband drivers with their whizzers. I was willing to live with it because the other 95% of the music I listen to is splendidly portrayed. Every speaker has its compromises, right?
This lively presence in the midband is also one of the reasons these speakers are so gripping in the midrange.
Recently, I was playing with the TacT and dialed down the response in the area I thought was to blame. The short story is these things sound great playing rock now. With this depression, they also sound closer to more-conventional speakers.
So, I think the Definitions' verve through the presence band is another reason some people don't love them.