I've reread this thread today, as I did read it originally when it was first started. Tough call I think, as this wasn't an on going issue with 600's owners. If it had been, then Axiom would had to have treated this matter like the auto industry, as part of a recall, I would think.
Earlier in my life I worked as a Industrial maintenance mechanic for thirteen years. Dealing in mechanical, electrical, and sometime electronic diagnostics and repairs. It seems to me that the mathematical probability that each of these five amp being defective, and being the sole cause are astronomical to say the least.
My training then tell me there is something else in play here about this particular unit, which leads to the amp failure.
If this was my situation, I would have asked for a replacement unit.
Axiom is well known for it's research & development, normally this means forward looking going out. But sometimes one must take a step backwards, to look at what has already been researched & developed. Now again I stated earlier I thought it was a tough call, because it was an isolated situation. Axiom was certainly not obligated in anyway in having to take back and exchange this unit. But I would have still asked, especially knowing that I've made numerous purchases from Axiom in the past.
Who knows, if asked, Axiom may had gladly taken it back to try and find an under lying problem with this unit. They could have possible resold it as refurbished (not taking a total hit) and making a totally satisfied customer as well. Making customers happy is what Axiom is well known for.
These are just my thoughts, I don't fault anyone on the handling of this particular matter. The OP as he has stated, has moved on, did a very nice deed in donating his Axiom gear locally. I hope he's enjoying his new HT equipment as well.
"When arguing with an IDIOT... make sure the person you're talking to... isn't doing the same."