To bridge or not to bridge....

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.

Weird, I started to have this same basic problem with a 9B. I went round and round with Byrston, my dealer and the speaker manufacture. Conclusion, I was under powered. I still think this was bull.... But hey, they know more than I do. The drop in speaker impedance was always in the back of my mind, just nobody said this could happen. They suggested I physically inspect the crossover, looks for burn marks, no luck. A 4b is a much larger amp, thou.

I doubt their is anything wrong with the amp. One interesting comment form Bryston was the their amps either work, or they don't. No middle ground. I also think their amps work great, in their design parameters. The 9b is stable to 4 ohms, not 3.9999 ohms. The speakers I am using stay under 4 ohm for everything under 650hz, a major fundamentals section of the spectrum. The 4b should be stable down to 3 ohms(their quote).

Also the "cleaner" the power I feed the amp the higher I could push the unit. Dedicated line, ps ultimate outlet and Richard Gray seamed to help raise the limits, but did not fix the issue. So I bought a Simaudio Titan, I no longer have an issue.

Another 4b is cheaper, but I once heard that a bridged amp never sounded "better" than an unbridged amp. Plus I don't know how much a 4b bridged would like a 4 ohm load.
Good Luck,
Marty

Thanks for all the info...
My Kefs 104.2 are rated at 4 ohms,92 db,can I not put a ohm meter across the posts,on one speaker at a time,and measure the actual ohms while playing at different volumes?
This would certainly help eliminate some of the guess work.
The red light appears at 1-2 o'clock position.Never do I go above 2 o'clock.
Why so loud you ask,when we entertain and are in the kitchen ,this volume is needed to maintain the party atmosphere in the kitchen,some distance away.
The volume is loud but not earth shattering.
My speakers are 16 years old..
Maybe a dedicated outlet...
I am going to sell the 4B ST and buy a 14B SST,for sure.
That is as good a place to start as any.Don,t you think??