RCA to XLR cables

I know they make cables for this but is the signal balanced at the xlr input? I've been told it's not. Is that a big deal?

What is a good source for those cables? Is Monoprice ok or does it pay to buy more expensive ones?

This would be going from the variable RCA outputs on a Rogue Cronus Magnum to the XLR internal amp input on the speaker which would be set to bi-amped mode. Speaker terminals would power the mids and highs.
rca to xlr cables give you the balanced connector at its xlr end but the signal is still 2 pole, single ended (one positive, one ground)

in this case, one of the three pins in the xlr connector doesn’t carry signal... but it lets you plug in to a component with that type of plug
Is it a major or minor loss of signal and/or sq vs xlr to xlr? One knowledgeable person told me other than a loss of 3db of gain compared to xlr to xlr, it should be fine and do the job powering the woofer.
Besides the drop in gain, the main thing you are losing is the common mode rejection that balanced cables (and connections) provide. In other words, any noise picked up by the cable is usually picked up by both the positive and negative phase conductor in the cable and cancelled out by the differential receiver at the destination end. 

Common mode noise in a home environment isn't generally a big issue unless you live next door to a radio broadcasting station or a factory with lots of big motors. And it probably has less impact on the bass than the upper frequencies. 

If the cable is constructed such that the conductor that is connected to the negative pin on the XLR connector is connected to the shield (outer part) of the RCA connector at the other end, then you are going to get at least some of the common mode rejection benefits anyway. 
XLR balanced is always more dynamic and quieter.

while i agree with you as a generality, 'always' is a strong word to use in this hifi world

many exceptions to common truisms

Not always only 98%.Thanks.