Who uses long interconnect like in dealers?

Mprime, You do have to be careful with cable selection to avoid RFI but unless you have unusual problem in your area good shielded cable should work fine. I would recommend that you audition the cable first. The only problem I have had is when I ran the cables parallel to power cords or near large power supplies - and thats easy to correct. I've never experienced RFI thru the cables.

I use 6 meters of Randall Research TX, which I recently
convert to balanced/XLR.

You rarely see long interconnects because of the wide spread use of equipment racks. I can't believe people continue to put these racks between their speakers.

Kana, how would you describe the sonic differences between the balanced run against the unbalanced?

Mprime- the noise floor was lowered, and output from the preamp increased,which lowered volume settings. Both of which improved low level listening pleasure.

Since the cables had been used for 15 years, there wasn't
any change in the basic sound qualities of the cables.

A nice improvement for $20. in parts and a little bit of
labor.

I never had an RF problem in my room with the cables in SE
config, but I think that's more a function of good cable and
equipment design than the length of the cables.

Suggest you borrow a long IC run, and see if it improve the sound of your system. I've run my audio systems with
long ICs and short speaker cables for the last 20 years.

I remember when preamps (all tube) had cathode-follower output stages with 600 ohm impedance. In such a circuit the coax connection to the power amp was critical. At the very least, hum pickup was a problem. Now, if you have a solid state preamp your output impedance is 10 to 50 ohms, and you can transmit that low-level signal a long long way: in particular, much further than you should try to drive the high-current speaker signal. Cheaper too if you buy the coax wire directly from the manufacturer, before it gets the absurd markup that goes with the designation "audiophile".

I run coax from the convenient eye-level shelves in an alcove where the source equipment resides, down to the cellar (that's 6 feet already) and along the cellar ceiling to shelves just under the living room floor where the power amps are located. Probably 30 feet in all. The speaker wires are just long enough to go up through the floor. I have never sensed any problem with this setup, and it sure cost less than the six! (L,Ctr,R, all biamped) 30 foot heavy gauge speaker wires that would otherwise be needed.