dap_pad: I know it's hard because I was in a very similar situation about a year and a half ago; trying to research and figure out which hi-end can I wanted to finally throw down my hard earned, high school cash on. However --and I've said it over at dapreview, and I've actually said it in this thread, I just don't think you're getting the picture-- I wish some one told me what I'm going to say right now: SOURCE FIRST!
You don't need $1k for a nice sounding rig, honestly.
It's so easy to get into the mindset that headphones have the largest play in the end quality of your system's sound performance because you come to a place like this to
find good quality headphones in the first place! Then after a while of reading about all the coolest and latest FOTM headphones you find out an amplifier is a MUST to make them sound good. Yet, we've never really learned what makes everyone's system sound so good in the first place because we've yet to really create a capable rig... We usually forget about our source so that comes as a last resort when we have extra cash on hand.
Most modern sources are going to be converting a digital signal to an analogue one that your brain can in turn interact with via your eardrums. This digital to analogue conversion is one of, if not the, most important constituents to higher fidelity. No matter how sensitive and well amped your headphones are they will always be reproducing a crappy signal if your DAC is crap to begin with. That said, most consumer grade audio equipment contain DAC's of poor quality, and/or have other elements to their output topology that fuddle with the sound.
So until you can get a cleaner signal from your transport/DAC to your Mini³/PK1, I'd wager any upgrade in the headphone department you currently make will only be a 1-5% upgrade. Sound of a higher accuracy is nearly impossible because your PK1's are a heck of a pair of buds to begin with. And they are probably sensitive/driven well enough by your Mini³ to replicate the sinusoidal signals your portable sources are sending them (to a precise enough degree) that any headphone more competent really couldn't expound a more faithful reproduction.
If any of this makes sense, buy a cheap home based CDP (Sony SCD-CE595 is a good place to start looking
, but you can also PM me for an even cheaper solution on par in quality), hook it up to your Mini³ and PK1's and just see where that takes you. Once the original signal is higher in fidelity you can think about reproducing it with more prowess (via better headphones), but all that comes later.
Once again, good luck on your journey.