I’ve owned the Studio 140 mk II, Response D28, and Response D38; and I’ve listened to the Response D1 and D2 at a local dealer.
The D2 is my favorite standmount speaker of any brand, and I would use the D1 in a room too small for D2. I haven’t compared the Tablette or 1SC to the D1 or D2, so I can’t comment on how they compare.
It sounds like everyone above has a good experience with the Studio 140 mk II. My experience was that treble was bit aggressive, and I preferred my move to the D28 which has an air-cooled tweeter that is the same (or very similar) to the D2 and D38. The D28 has smoother treble IMO than the Studio 140 mk II, but still somewhat sharp in my room. Oddly, I have never had the problem with too much bass (with any ProAc), which some people experience. The D28 has a warm cabinet coloration which sounds great with acoustic bass.
I eventually moved to the D38, which feels like a significant step up the ProAc ladder. It has less cabinet coloration than the D28 (which I actually miss), deeper and tighter bass, and the treble is about perfect for me. I would say the midrange stands out a bit more but not in a bad way, perhaps due to the extra mid-bass driver or due to the height of the upper mid-bass driver. The D38 doesn’t sound compressed at higher volumes, yet has excellent resolution at lower volumes. The size of the soundstage is life-sized, and more realistic than previous models that I’ve owned (image firmness and tonality are improved upon).
For anyone contemplating ProAc, one of their strengths is imaging (very spatial, almost holographic), but they can be fussy with pairing of electronics and cables. The good thing as well as the bad thing about them is that they will show the faults of everything else in your system, but I like that characteristic. Some people may feel it’s because they’re not as balanced or neutral as other brands, but I feel it’s because they are revealing and I find that a good trait. And although they may sound warm in some respects, they can also sound forward. But I've found that I prefer this quality, because they sound good at lower volumes (the D38 also plays loud just fine).
If you like ProAc, the next question is what type of amplification and speaker cables are you using? I’ve heard that they sound their best with tubes or Class A solid state, but I haven’t tried that combination yet. I’ve been using Simaudio integrated amplifiers (I-5, i5.3, 600i, 700i). While perhaps not the best combination for ProAc, I am fond of the spacious and somewhat holographic sound of Simaudio, which shares some of the same traits as ProAc, IMO.