What cables to use.

Optical cable will outperform hdmi in long distance application. I have 30ft long hdmi cable connecting my main TV with smaller TV in my tool room in my basement and the quality is pretty low. When I tried 5ft hdmi connection just to check difference,  it was significantly better. 

15’ should not be a problem for a decent HDMI cable.

I’ve used a 30’ Monoprice a few times to directly connect our Internet modem to the cable box (usually run WiFi, but it craps out from time to time) and it worked fine.

This said, our previous and current TV cap out @ 1080P, and like I said I’m not picky about TV (it just needs to look/sound OK).

DeKay

Plus 1 dekay 

Amazon has a 15’ hdmi for $16.99 with a 10% off coupon that should do the trick.

4K HDMI Cable 15FT,Highwings 2.0 High Speed 18Gbps HDMI Braided Cord-Supports (4K 60Hz HDR,Video 4K 2160p 1080p 3D HDCP 2.2 ARC-Compatible with Ethernet PS4/3 4K Projector Game Monitor ect-Grey https://a.co/d/ihUojrE

All the best.

I'm using an 15meter 8k Fiber Optic HDMI 2.1 cable from my Oppo BDP105 to my TV.  Works great.

What brand and type are your powered speakers?