Balanced in phono stages preamp?

Depends. Many products convert balanced to single-ended after the input and then process the signal in single-ended mode. Other products are single-ended all the way thru and then have an additional stage at the output that provides a balanced out. In either case, it is unlikely to be advantageous to use the balanced in or out, as compared to the SE inputs and outputs on the same piece of gear. Only a few phono preamps are truly balanced from input to output. AFAIK, these include BAT, Ayre, Aesthetix Io (in spite of what someone wrote below), and Atma-sphere. In my opinion, there ARE some advantages to a balanced circuit in the latter case, noise and distortion cancellation being two of those advantages. Before purchasing a "balanced" input phono preamp, it would be wise to make inquiries as to what is really going on inside the unit. I think a lot of the reason folks are indifferent about the advantages of a balanced circuit is that they have not really heard one properly implemented.

Your conception about what is happening to the output from the cartridge when interfaced with a balanced input is flawed, insofar as you see it to be a negative. The signal is not "cut in half" per se, and there is no conversion of the signal from "two wires" to "three wires". The ground in a balanced input is independent of the + and - inputs. The cartridge sees only the + and - inputs and does not know the difference between +/- vs hot/ground in an SE input. One could make an argument that the output of a cartridge is inherently balanced in the first place; the choice of hot and ground is arbitrary, really.

However, your impression that your BAT phono sounds best with an SE input is not one I would challenge, since I don't know the particulars of that unit and you are entitled to your opinion. You might discuss your finding with BAT, however. Nor is it my intention to argue that all truly balanced circuits are better than all SE circuits, not by a long shot.

Not to beat a dead horse, but you wrote, "Lewm - didn't mean to say that balanced was bad just that there were trade offs." Not only did I not say that balanced was bad, but I was trying to say that I prefer it and why. Also, I was trying to convey the idea that one must be cautious in interpreting what manufacturers claim about their equipment, especially now that balanced circuits seem to be in vogue as never before. Try to find out whether the circuit inside is truly a balanced one (if you care about the issue at all), because in a lot of gear an XLR input or output is derived from what really is a single-ended circuit internally. Many CD players pull this trick by adding a chip at the output, which not only is a phony way to do it but also will likely degrade the sound a bit compared to the SE outputs on the same piece of gear, due to the added active device which is typically a middle grade op amp, at best. Even my former Accuphase DP75 (a $10K cdp when new) and the Sony SCD1 used this approach to obtain a balanced output. For this reason, when someone says they don't hear any advantage to a balanced output, I have to first wonder what they have been listening to.