Pani, I am fortunate enough to have heard both the Dart and the CAT JL2 and JL3's. The Dart is a great ss amp, the CAT's are great tube amps...not really the same presentation. Personally, I prefer the CAT sound....the Dart is slightly-slightly more mechanical sounding. The CAT amps gave me the impression of more power on reserve than the Dart. I heard both of these amps on two different speakers....1) a pair of Magico Mini 2's and 2) a pair of ML Summits.
Front ends were the respective CAT and Dart preamps with a TW analog rig as source.
If one had to choose between the two, it would be a pretty difficult choice, as both are VERY good....my slight preference is to the CAT.