The subpack is a little of an issue for me too with the line-out as well.
There's a guy on the iaudio forums who's making some cables that allow you to completely bypass the subpack and cradle. He's making some for me and I'm hoping to get them in the next two weeks. He was waiting for Cowon to ship the AC adaptors.
The headphone out is very decent but I also use the line-out for my amps. I was using my Porta Corda Mk I for awhile and while I was happy with the quality, I wanted to see how my SuperMacro 1.0 would compare. The soundstage was much better and the music seemed warmer to me personally so that's what I'm using until my 3.0 comes in. I've been pretty exclusive with my super.fi 5 pros on the X5 and I'm pretty happy with the combo, however it works well with my HD580s and SR225's. I haven't tried them with my DT770/80's yet; maybe today. Anyway, this should last me for a few months. On a tangent, I don't know what I want to get next. I'm thinking of the K701's (but I'll need a home amp for those, and a new deck, and a new wallet, etc etc etc), but I'd like to upgrade the super.fi's - I can really see myself getting some e5c's or maybe some ue-10's too). Anyway, I'm getting off track.
I'm also very frustrated with the limited number of cases for the X5/X5L too. Noreve has some decent ones, and zCover too, but that's about it. Misticaudio was supposed to have had some, but their webpage is down so I don't know. Then there's cowon's own case, and that surprisingly substandard. I'd love to get a type of MicroStrap for my SM/X5 rig, or a Vaja case (i know they're a little pricey, but i think they look so slick), or anything else that's exclusive for the X5. I think I'll just resort to a camera bag and one of those livestrong bracelets for my $1000 rig