What are the drive requirements of your 811 amplifier? Do you suppose you inadvertently over drove it? There has been a recent thread of having Flex set the power to lower values as a default. Several have complained that it defaults to 100 Watts and that might have been your problem. You might end up with the same problem with any other amplifier that requires less drive.
Interesting read about the 811-A:
I have tune power set to 20watts and power out set to around 60 watts in each band profile otherwise the amp' s micro complains of overdriving when I tune. The Acom 1K will run 1200 watts but shines at 1000 watts all day any band any mode 6-160. 1K on RTTY is nice! Also will tune up to 3:1 SWR. Would do it again, price point is right for me any way.
Since I'm shopping for an amplifier and am looking at the ones mentioned here, I've been following this thread with great interest. My major concern is the transistor vs. tube question that YV5WZ raised at the top. I've had tube amplifiers in the past (including one I built from scratch in high school back in the early 1960's) and never damaged a tube.
On the other hand, I've blown the output transistors in my 6300 twice. (Fortunately it was under warranty both times.) I don't know how much these transistors cost but FlexRadio support said they were very expensive. (No one at FlexRadio seems to know what caused them to blow. FlexRadio finally said that their ATU was not able to deal with my antenna's reactance and suggested I buy an external ATU. I bought the Palstar HF-Auto antenna tuner and haven't had any problems since.) Furthermore, I can replace tubes myself but probably not the transistors.
As I see it an RF transistor amp has these advantages: 1) No warm up required, 2) usually have protective circuits, auto tuning, and auto band switching, 3) lower power consumption and generate less heat.
Tube RF amplifiers have the advantages of: 1) User replaceable tubes (Adcom's warning notwithstanding), 2) tubes can be purchased from several vendors, 3) cost per watt is lower, and 4) high power tube RF amplifier design is well tested with many decades of use.
As much as I'd like the instant-on and CAT features of the Adcom 600S, my fear of blowing output transistors and paying high repair costs have me leaning toward the similarly priced but higher power Adcom 1000. Is my reasoning flawed? Is there any data on high-power RF transistor vs. tube reliability and lifespan?