Hi all,
On the specific point Bertie asked me about, the following *may* help. The WINGPOON DAC was perfectly solid insofar as the soundstage was well reproduced (nice and wide); instrument spacing was positively good; trebles were ok without being bright (although a little grainy); mids were very well reproduced and the bottom end was there although tending to be a little soft. In my opinion, few would spend £20 for a WINGPOON and think it was a rip-off; especially if it was being used as a mere stopgap whilst trying to find something more long term.
When demoing the Marantz PM6005's onboard DAC, I did an A-B with the WINGPOON and the latter definitely sounded duller by comparison. It may be helpful to note that I do not like a crashy, bright sound so I don't regard the PM6005 as being "bright" (at least not in the set up I tried). Apparently, the PM6005 *can* sound bright so it seems pretty clear that speaker and cable combos matter.
As for the overall "should I get an external DAC?" conundrum, it's down to personal preference in terms of any combination of the sound, the physical system set-up and/or the way in which you use your system(s).
One initial distinction to bear in mind is whether the driving force behind system changes is system tinkering in itself or actually listening to music. For me, it's the latter.
At one point, I had a fairly romantic view (misplaced!) that "...it would be great to make the X10 "high(er?) end" by adding an external DAC

. Once I'd established that £200 for a DAC didn't necessarily provide aural nirvana, I started to think somewhat more realistically

.
In the end, the decision to go for the integrated amp / DAC combo (PM6005) was down to a) the sound being (more than) good enough and b) it kept my system to a smaller footprint. Obviously, the X10 + speakers is an even smaller footprint but I have another digital sound source (TV) and preferred my overall set-up to include an external amp (& DAC) for flexibility.
Dortmunder: most of the high street chains (e.g. Richer Sounds, Sevenoaks, Superfi, Peter Tyson) stock an X10. I'd definitely do some demoing, playing music files from a USB stick through the X10, trying various combos of standalone DACs + amps as well as integrated amps with onboard DACs. Definitely ask for A-B tests. If you have phono/interconnect cables familiar to your ears, take them along so that standalone external DACs can be connected to separate amps on your terms insofar as is possible. Speaker wire and speakers will be variables so matching those items in the demo will be trickier

.
The downside of dealing with the above mentioned stores is the huge variability in the quality of advice. I got lucky when demoing the PM6005 but other experiences have been very firmly filed in the vault marked "shockers"

.