Are Expensive Balanced Interconnects Necessary?

That's not at all how a differentially balanced circuit works. A differentially balanced circuit uses both an inverting and non-inverting signal at its input, then amplifies the voltage difference between the two signals to produce the output.

Depending on the design of the balanced circuit, there can be substantial benefits to using it in balanced mode, rather than SE. For example, with ARC balanced preamps, you'll get 6 dB more level using it in balanced mode. That means 6 dB improved s/n.

Maybe when noise is zero… @cleeds ?

But the idea of CMRR is usually more than 6dB.
Let’s look at each side:

• (+) S + N
• (-) -S + N

(+) - (-) = S + N - (-S +N) == 2S 

Balanced cables quieter and more dynamic.

In my experience it depends.
For example, the Pass Labs preamps and amps sound better with XLRs. Also, using XLRs with these components diminishes the audible difference between cables. I have experienced the same with Audio Research LS and Ref series preamps, as well as ARC CD players. With my Benchmark DAC3 HGC in my system I prefer single ended to preamp. I run a 15ft long Mogami Studio Gold XLR between Pass XP-12 preamp and XA-30.8 amp that powers my Wilson Sabrina. I compared it to my 1m Acoustic Zen Absolute Copper and yes there’s a slight difference but the AZs aren’t necessarily better and the difference in this case isn’t drastic. 
Now I also had single ended Acoustic Zen interconnects and each step up the line brought a pretty significant improvement. There’s no substitute for trying though. 

I'm thinking of moving the linestage along with the sources of course further from the speakers. 

I'm currently using 1m Nordost Red Dawn XLR between the preamp and the power amp. (This option works definitely better than any RCA I have tried).

If I move it, I'd have to get 2x6m of balanced cables to reach the power amp. I guess the length will not make a difference in this case but what about the cable itself? I'd build something myself just because of the length. Maybe something like Belden 8402 or Mogami W2534 as Paul from PS Audio suggests. 

https://www.psaudio.com/ps-how/how-to-make-a-50-pair-of-high-end-xlr-cables/

I don't care if I pay $2 or $20 for a meter. The Mogami cable is super cheap.

Do you have any recommendation for something better? 

Thanks

Depends on what you mean by expensive.  Having a balanced preamp to amp as well as two balanced sources the differences between cables is nearly identical to the differences in sound for single ended cables.   

I tried basic balanced cables including Benchmark and two pro cables but found them to not sound as dynamic or clear as the Van den Hul hybrid and Nordost blue heaven cables.  

They make the same difference and are worth the investment for me.