The Amp1mkII (stock) is already a very capable performer - clean, balanced, musical, fairly 'neutral'. Balanced power is similar to the Amp8, from what I've been able to gather. Amp8 is a touch warmer, bit more 'oomph' and a bit more bass body. The Amp9 is also really nice, neutral but musical with a solid bass and midrange (it's the tiny tube, yo), but it is a good bit noisier with sensitive IEMs, even in low power/low gain mode, and the tube 'rings' a bit if you bump the player. In high power mode, the noise is way too much for me for my Campfire IEMs, in low power/medium gain it is audible, but not offensive at all unless you're listening to very quiet music. But the Amp8 and Amp1mkII are quieter.
In terms of power, the Amp8 will easily drive all my full-sized headphones. The Amp1mkII single ended output I find a little anaemic for both the Sennheiser HD6XX and the Hifiman HE4XX. Volume is fine, but they're not driven all that well, and you need to push the volume levels pretty high. Amp9 does better in it's high output/high gain combo mode. But the Amp8 is much quieter and just as musical to my ear.
As a reference, Amp1mkII or Amp9 single ended volumes, medium gain, Andromeda were around 90-100 for a-bit-too-loud-to-be-healthy, and my discomfort threshold for Amp8 is around the 45 mark. With maximum volume being 150 (android mode).
Honestly, I'd say give the stock Amp1mkII a try first - its a very capable performer. I just figured I'd try out the different amps considering I have a player that allows amp swapping