You are quite well served in terms of amplification, then.
My perception of the issue nowaday is this: the K1000s are fantastic on a number of aspects, but not on others. To me, its tonal balance is significantly tilted towards the midrange. So it takes a lot of time and research to find an amplifier and source that are capable of minimizing its downsides and make you fully appreciate what it does well.
So while your amplifiers are very very good, they just might not be exactly what the AKGs demand. But well... I guess I should leave that to more experienced people :)