Tboooe, of the pairs you've suggested I can only say that I own the X1, and have heard the x350.5 (but that was not in my system). The X1 is an awesomely quiet, very neutral preamp. Nicely extended in both directions, with a very nice midrange. It is one of the best preamps I've ever heard. There is another thread where it was remarked that - in comparison to a great tube preamp - the X1 mids are thin and bare. Frankly I can't agree. I'm not sure how much system dependency the X1 has in placement with other components, but in my system (driving a Van Alstine FetValve amp) I get a satisfactory midrange indeed. Tonal colorations, a real organic presence.
My recollection of the x350.5 is that it is also a fine sounding piece of gear. It was being driven by the X2.5 and I thought the pairing sounded very musical. Maybe just on the shady side of neutral. I can't remember what source was being used, though.
My only complaint with the X1 is that the gradation between the volume click stop positions is too large (about 1.25 dB).
