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I have just bought a pair of Shure SE530 in ear monitors
(earbuds). Absolutely superb (albeit overpriced). In
fact, I'm finding I like them every bit as much as my
trusty 15 year old Stax "earspeakers".

Interesting. I've heard Stax earspeakers, and frankly I don't know what all
the fuss is about. For example, Sennheiser HD580s sound about as good to me,
maybe a tad smoother.

My music is all on my mp3 player and PC these days. I
ditched all my expensive gear a few years ago when I
finally admitted to myself that my ageing ears couldn't
tell the difference between 128kbs mp3 and the originals.

That of course depends on the MP3 - some music can be coded accurately
enough at 128 kb, and others can't.

Now I will also happily admit that my little Sony flash
player with the Shures sounds every bit as good as
anything else I've ever heard through headphones.

Well, certainly as good as anything you've heard lately. I suspect that if
you could get the ears from some 14 year-old that took good care of his, you
might hear things a bit differently. ;-)

Now I want to be able to plug my Shures into my PC.

What's wrong with the green jack on the PC?

Don't knock it until you try it!

But if you try it and find it lacking, it might be because it simply does
not get loud enough.

There's a little device called a "Boostaroo" that gives a 6 dB gain, which I
find helpful with some earphones. The price in the US is under $30. When I
put it on the test bench, it was very, very clean, conservative specs
notwithstanding. I circumvented the battery problem by making up some AA
battery simulator out of dowels and brass screws, and power it with an old 5
VDC power supply from a cable box that was dustbinned years ago.

What I want is a little desktop DAC/headphone amp with volume
control that accepts either optical or USB iputs from the PC.

IME PCs with optical digital outputs for audio are very rare, but USB jacks
are very common. If you want to spend some money, there are a modest number
of USB-based recording interfaces that also have headphone jacks on them.
The quality is going to be pretty good, but you're going to pay for a lot of
hardware that you may never use.

Here's some example:

http://www.m-audio.com/products/en_u...kUSB-main.html

http://www.m-audio.com/products/en_us/Transit-main.html

http://www.emu.com/products/product....roduct=1 7511

The people on Headfi forums - headcases, mostly - are
well meaning but many have lost the plot and obsess about
things like thickening of decaying transients in the
lower mid registers etc etc

If the discussion was rooms and speakers, yes. If the discussion is
headphones, then its more likely that there would be a thinning, rather than
a thickening.

Does anyone have any advice on something that is
reasonably prefessionally designed and built and will do
the job? Prices vary from £50 (for the Headstage Lyrix)
to the price of a family holiday, all for a bag of bits
that'd cost about £10 from Maplins. Then there are
professional USB soundcards like Behringer, E-MU etc,
with lots of flashing lights and connections, but
apparently "poorly designed headphone outputs".

It's really hard to have a poorly designed headphone output, other than to
not have enough loudness.

On a related point, these "Headfiers" plug in the
headphone output on their mp3 players into these little
amps to power their headphones. What is the point of
that?

The headphone jacks on MP3 players are generally built on the weak side to
keep kids from blowing their ears out.

Surely you can't improve on the signal coming out
of the headphone socket?

Actually you can. There are a number of possible directions:

(1) Just plain louder.

(2) Incorporate some kind of HRTF processing to make the subjective
experience more like listening to speakers or live.

(3) Add effective equalization to touch up the natural variations in how
various headphones interface with people's ears. At that level, ears can
vary considerably.

I know bugger all about
electronics but it seems that the most this would achieve
is literally just amplify the signal, by which I mean,
just make it louder?

That is one of three general kinds of possibilities.