50' speaker or 50' interconnect?

Ocos speaker cables are supposed to be good for up to 100 meters.
http://www.dynaudio.com/eng/systems/lines/accessories/ocos.php

If you are running balanced interconnects, you need to make sure your equipment is also truly balanced to reap the benefits of balanced. An XLR jack does not mean truly balanced, necessarily

What preamp are you using, and if you can readily determine it, what is its output impedance?

The higher the output impedance, the more critical cable capacitance will become. The symptom of cable capacitance that is too high in relation to output impedance would be upper treble rolloff, and its associated subjective effects (sluggish transients, dull sound, reduced ambience, etc.).

I can do some quick calculations for the Mogami once I know the output impedance of the preamp.

Here is a link at which you can purchase a 2534-based 50 foot length of terminated Mogami Gold Studio xlr cable, for $98.95, from B&H Photo Video (an excellent seller). They have a 15 day return policy:

http://www.bhphotovideo.com/c/product/318716-REG/Mogami_GOLD_STUDIO_50_Gold_Studio_XLR_Male.html#features

Blue Jeans xlr cable, based on Belden 1800F, has a bit under half the capacitance of the Mogami, but does not provide the Mogami's quad conductor configuration which improves noise rejection. Some of the higher end cables, such as some Cardas cables, also have considerably less capacitance than the Mogami's, but of course will be very expensive in that length. So I second the suggestion of Mogami unless the output impedance of your preamp is particularly high at high frequencies.

Regards,
-- Al