To bridge or not to bridge....

Yep, the truth is there that your unit needs to be checked.
Good to have 20yrs transferable warranty though!

Bryston replied "very unlikely that there is a fault.Amps usually work or they don.t.
I guess I'm under powered or need hearing aids.

First of all how hard are you driving the system? Do you turn you volume above 3 o'clock?
Did you check if your speakers can handle 600W/ch(I believe that bridged Brystons 4b-st through such wattage in 4Ohm or even more)?

I've owned the 4bst and 7bst and found that the former could play very loud without red lighting. I liked the sound and punch better with the 7bst regardless of usual listening volumes. I can't recall turning my preamp up more than the 1 o'clock position since it got really loud much before that.

It didn't occur to me before, but you said that the problem is only recent. Two questions: 1) Does your orange (or is it yellow?) light come on before the red does? 2) Does everything sound the same before as it does now? Maybe your warning light system is bad. If you are listening at the same volumes (both position on the dial and perceived db level) as before and the only difference is that the red light comes on now, maybe it's the light. You might want to ask Bryston about that. Also, you might want to carry the 4bst to a friend's place or your dealer and plug it into his/her system. See if and when the light comes on there? If the amp's OK somewhere else, maybe the speakers have a problem and the impedence has dropped. Just a few thoughts.

Check your AC voltage, it may be too low.