Agree on the possibility of a 4 tuner issue - found some unanswered posts going back asking a similar question to what I am asking!
As regards replacement tuners for my current setup, its not really an option.
I have the hybrid tuner on the ET8000 configured as dvb-t (did use it as a dvb-c for a while as well!), and all working fine.
My trusty CT9000 (which got converted to an ET9000 somewhere along the way) got a bit of a power wobble, and does not see the sat tuners anymore - boots up okay, sees the hdd and usb tuner, dvb-t works fine locally, sat works using fallback configuration to the ET8000.
Suspect its a psu issue, and have changed some capacitors, but nogo. The power issue also damaged lnbs and a disecq switch.
The sat tuners are soldered onto the mainboard, so not really replaceable!
In looking at a replacement, came across unicable and fbc tuner's, so will probably go down that route, especially now that I understand a bit more on how the 6 leaf tuners can be configured across the root tuners, and how the legacy outputs work on the unicable lnbs.
Too much time on my hands....