The Atma-Sphere MP-1 and MP-3 are both available with optional RCA outputs (in addition to the dual balanced outputs). When this option is added there is also a switch installed to switch between single-ended and balanced operation. This is needed as otherwise you can encounter a buzz. When the switch is in the single-ended position the output of the preamp is then single-ended regardless of the connector used.
You can also use and adapter or cable that does the conversion. If so, it is important that pin 1 and pin 3 of the XLR be somehow connected together. This will not harm the preamp in any way and is needed to prevent buzz.
The reason the buzz can occur is that the output of the preamp (pins 2 and 3 of the XLR) is a true balanced circuit, and therefore is 'floating' with respect to ground. If only one side of the output (for example pin 2 of the XLR connection) is used, it will behave like an open input, because that is exactly what it is. To solve this, one side (usually pin 3) is tied to ground (pin 1).