Don't know about the 9550 as I never owned it.
One of the first things I did on my 9560 was install custom drivers as the stock drivers really colour the sound no matter what I did.
And there was the pop up issue. Don't know how it is today but after a week of figuring out what to do, I haven't ever looked back.
Indeed lack a bit of power at times, especially as I'm using Joe Bloggs' magic pixie dust thingys and bumped the gain down about 10 db to prevent clipping.
I never really minded listening out of the headphone jack unless it's with something sensitive. It gets quite noisy with those but they are alright with other stuff. Hate the speakers of course.
I guess the only real complaint I have with using headphones on the XPS is it has a very loud pop when it starts up, so I have to unplug it before boot ups. Apparently many people have that issue whenever audio plays when using stock drivers but I've never bumped into that with the custom drivers I use. These custom drivers does not support the mic in the combo jack though so not sure everyone will be happy with them. There are other solutions and hacks I've seen pass around but I don't know if they do anything about the colouration of the sound. :/