Looking at running a Magnetar UDP-900 blu ray/4K player, both in home theater and audio system. This would necessitate a 50' HDMI cable. Can anyone recommend a good quality extender?
No, but I can recommend the ADD Powr ViviD, a new video conditioner for HDMI and coax signals. It will work wonders on HDMI 2.0 and 48 Gbps frame rate. Simply astonishing product worth checking out.
I do not have sufficient experience with different brands, but 50' requires an active (powered) HDMI cable. I would look at Blue Jeans Cable, a respected cable company: https://www.bluejeanscable.com/store/hdmi-cables/hdmi-cable.htm A 50' active (Series-3) HDMI cable from them is $85 USD and handles 18 Gb/s data.