Really Long XLR

Try https://www.bluejeanscable.com/store/balancedaudio/index.htm . I have ordered long (~20’) XLR cables from Blue Jeans Cables and they work fine at a reasonable price. Custom lengths are available.

I use the low cost Benchmark XLR cables for long distances. The longest they have is 25-feet but they likely can make longer. I need to buy 2 more long XLRs and waiting until Benchmark stock is back for the 25-foot length.

My other XLR and RCA cables cables are from WyWire and Audience

The cables between my AES-48 compliant preamp and amp do not make a difference with various XLR’s I tried.

Even with my non-AES-48 compliant preamp and amp the Benchmark XLR sounds great.

I keep my gear away from the speakers in both of my 2-channel rigs. Only the amp is close to the speakers.

I’d use wall/floor mounted plugs and run Mogami or Belkin. This way you can keep the cabling 100% out of the room.

Otherwise, make your own with Mogami or Belkin, shielded is a must at these lengths, and Neutrik connectors.

Besides Parts Express, Parts Connexion (for connectors) you should see what Markertek has.  They often have good discounts on Mogami.

+1 to what Erik listed. Since you're willing to build your own, order some Belden cable from Raw Cable, and Neutrik connectors from Markertek.

I would make my own cables from 16 awg Duelund DCA wire.  The wire for a 40' set will cost you $400.  A good set of XLR connectors is $100.  Sheath material another $100.  If you don't have silver solder, another $15.

Jerry