On the topic of the F1, it may not statistically be the best amp for the K1000's. However comparing a ~4.5 cm headphone driver to an 8 inch driver and ultimately trying to make the case that 10watts isn't sufficient to drive the k1000's is ridiculous. The output impedance of the F1 is 80ohms. In a perfect would it would be 120. The phase mismatch is fairly insignificant. While the sensitivity is more important one can't forget that despite its 74dB/w rating, it's still a headphone, and an extremely nimble one at that.
While the K1000's have a 74 dB/w rating, we cant forget, they're still headphones.
In defense of the F1 it does certain things that most other amps aren't nearly capable of. Realism being the number one trait. Inherent to its simple design, the F1 practically eliminates the noise floor. The result is a presentation of sound which appears into space and disappears back into space effortlessly. While the Ram Audio BUX 2.0 which haven't heard (or heard of prior to this thread) likely controls the drivers better (hard to imagine) and may excel in other areas, its complex circuit makes this quality of the low noise floor impossible (not to be confused with a black background). The effect of this, in my best estimation, is a loss of this sense of effortlessness. The tradeoff -- stronger dynamics. Please correct me if I'm wrong.