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).
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).