For pretty much the past 6 weeks it's been the iPad1+HP-P1+Pico Slim+Ultrasone Ed8's. When I first bought the HP-P1, I wasn't all that impressed with the amp as it sounded dark and the highs were veiled. My friend loaned me his Predator (which I used as the amp) and since then I had used the HP-P1 almost exclusively in DAC mode only. I immediately went out to buy a Pico Slim soon after as I couldn't borrow my friends Predator forever.
As Jude said, if one were to use the HP-P1 with an external amp, then you're better off with a CLAS since DAC2DAC, the CLAS is a smaller form factor, so I picked up the CLAS after the Tokyo Headphone Festival from Ken/ALO.
What I didn't realise was the HP-P1's amp just needed more time for burning in to open up. So recently when I went back to just using it as a DAC/Amp, I was surprised it has improved so much - despite me using it only as a DAC-only, I did inadvertently burn it in over the past few weeks. The difference between the HP-P1 DAC/Amp now and HP-P1+Pico Slim has narrowed considerably and if I knew then what I know now, I probably would have been happy enough with the HP-P1 and didn't need an external amp.
One thing to know though is that there is still a difference of the HP-P1's amp vs the Pico Slim, in that the soundstage with the Pico Slim is still a little wider and deeper. However, at this point, since the gap is narrowed so much compared to a month ago, it's merely a question of personal taste and budget if that difference is worth a Pico Slim (6 weeks ago, I'd say yes...but now, not so).
What's my personal final setup? Once I get my UM IEMs back from repair (and new ones made) I'll test again but I think there's a very high chance I'm going for the CLAS+Pico Slim setup for now, and for the Rx MkIII Balanced instead (and get my Ed8's balanced too). Portability is not an issue for me as I have another alternative for portability (the Go-DAP 4.0 + iPhone 4S). Therefore the iPad+CLAS+PicoSlim/RxMkIII+Ed8s will just get thrown into my satchel.
From what you've written before, if you want portability for your iPod, the only official way is the HP-P1 but I'd say give it a chance to burn in to get the most out of it. If you opt for SQ over portability/price, then the CLAS+whatever amp you choose.
P.S. I wrote "official" 'cos there are ways to interface your iPod/iPad/Touch with the VentureCraft Go-DAP 4.0 and I've done it myself with just a very simple dock extension cable. This is also extremely portable - but officially, the Go-DAP is meant to be for iPhones.
P.P.S. I'm also trying another unofficial combination of Dock->USB converters to try to use the iPad with a -standard- USB DAC. This is to bypass Apple's 20mAh limitation on the iPad. And if this works, it expands the choices of USB DACs considerably.