Now, there isn't much new about the x-torq, it just is Huskys name on "strato" saws, that has been around for over a decade by now. Actually Stihl only can use M-thronic (which is a version of the early Husky Autotune) and reportedly some Husky owned "strato" patents just because Husky lets them do it (I assume there are royalties involved, but don't know it as a fact).
Regarding the saws you list, the 661 is a "strato" (x- torq) - the 390 and particularly the 395 are much older designs, that are not.
All I can say about the 661 is that the specs are impressive, but the model has had a very troubled start on its "career" - like being called back etc. I am no expert though, just going by what I have read (in this case). I understand it is bad enough to have moved many PNW loggers from Stihl to Husky in the 90cc class.