Assuming your output from the TV is fixed level:
Add an external input selector switch and run the cable box and DVD straight to the Bryston this way. Make sure the cable box is on fixed level out.
Most TV's only put out 150mV or so line level and your CD player, for example, is around 1V, so big level difference. For some reason TV manufacturers haven't figured out what line level means in the 21st century...
If you had an AV receiver, you could just tweak the input level to match other sources, but in your case you cannot.