If the long speaker cable alternative is chosen, IMO there is a question which has not yet been raised that is essential to selection of an appropriate cable. And that is the impedance characteristics of your speakers at high frequencies.
I did a quick search but was unable to find an impedance curve for your speakers. Perhaps you can obtain one from MBL, or from a dealer or distributor, or by searching further with Google. If the impedance at upper treble frequencies does not rise substantially above its 4 ohm nominal value, then the inductance of the 20 meter length of cable becomes a significant issue. Relative to 4 ohms, the inductance of your present wires, and the inductance of 20 meters of Anti-Cable, and the inductance of ordinary heavy gauge wire, will all very likely be significant enough to perceptibly dull the extreme upper treble, and to cause perceptible sluggishness of sharp transients.
A major reason that the OCOS and Goertz cables that were suggested above tend to be good for long runs is that they have extremely low inductance. However, they also have extremely high capacitance per unit length, which would be factored up by the 20 meter length to levels that MIGHT be problematical for some amplifiers.
IMO, if you determine that the impedance of the speaker at upper treble frequencies is not considerably higher than its 4 ohm nominal rating, and if you settle on the long speaker cable approach, the Kimber 8TC that was recommended by Elizabeth would provide an excellent combination of low inductance, reasonable capacitance, and low resistance. The 12TC she also suggested is even better in those respects, but I suspect that it may be overkill.
I also agree with the comments that if your components provide balanced xlr interfaces, and assuming the sonic performance of those interfaces is good (which is not always the case), the long interconnect/short speaker cable alternative would be highly preferable. If you have to use unbalanced rca's, Bryon's suggestion of splitting the difference is also a good one, if feasible.
Regards,
-- Al