Running unbalanced, high impedance, low level signals like a Rhodes or guitar that far will lose you quite a bit of high frequency while picking up plenty of noise - you could use a passive transformer DI box, but their input impedance is typically around 20k ohms - I've never measured the Rhodes outputs for impedance, but some passive guitar pickups have impedances up around 100 k ohms. You don't want to load such a pickup by using lower impedances, because it will drastically change the tonal quality (not to mention noise with longer cables)
My recommendation for what you want to do would be to use an ACTIVE DI box, such as the top one here -
http://www.whirlwindusa.com/dirbox.html
Then, on the end where your amp is you can convert the high level, low impedance balanced signal out of that box back to high Z unbalanced for your amp with one of the less expensive devices, like
the IMP 2. Its unbalanced side has a 20K impedance, which won't be loaded down by any guitar amp you plug it into.
The above method won't TOTALLY keep from changing the timbre of your guitar or Rhodes output signal (nothing is that good) but it's the best (not cheapest) way I know of.
IF you just use a pair of matching transformers, one on each end, their lower-than-guitar unbalanced side impedances will change your tone a fair amount, and not for the better. Still, for about $30 you could try a pair of the "little imps" or equivalent, and see if you can live with it.
Hope that helped... Steve