I agree with George. You would be spending a ton of money to purchase a preamp that would do justice to your existing equipment, and if it provided any benefit at all it would probably be the result of either providing a more transparent volume control mechanism (which I doubt, given the apparent quality of the CD-5), or masking an existing deficiency in the system (which would be best addressed, if present, by determining the root cause and correcting it). And at the same time it may reduce overall transparency unacceptably, and it would probably result in too much gain, as George indicated. The latter issue forcing you to either use the preamp's volume control undesirably close to the bottom of its range, or to use it in conjunction with a lowered volume setting on the CD-5, which would negate what may be the main potential benefit of adding a preamp.
Regarding the impedance question raised by Reynolds853, both the 30 ohm output impedance of the Spheris and the 300 ohm output impedance of the CD-5 are fine matches for the X350.8.
Regards,
-- Al