while people seem to react against Y splitters, I haven't been told
exactly WHY they are such a bad idea.

Is there a good electronics engineering reason to avoid them? Would it
make any detectable difference in the real world (double-blind testing
etc etc)?

Frankly, I've used "Y" splitters many times both in and out, and have never
had a problem. I try to make sure that I have only one piece of the "Y"
active at any time, as your say you do. Theoretically, I suppose you could
have an impedance problem if both pieces of gear receiving the signal had
marginally low input impedances, but I've never experienced that in the real
world.

Marvellous! Thanks for the feedback.

I have found, since delving back into this hifi business after some
years of not taking an interest, that much of the info you can find
(in hifi shops, in reviews, even in newsgroups!) is pretty clearly
total tosh. Recieved wisdom, as it were. There are a lot of charlatans
about, a not a lot of rigour!

Upgrading my current system is proving an interesting experience. It
is tricky to work out where the longest levers are for actually
getting a better sound (rather that making things look prettier, or
emptying the wallet). I reckon money spent on expensive cabling is
probably largely wasted (and it's the first thing a lot of people tell
you to spend on!). I know the first thing I need to do is get the
signal away of the DJ mixer (unless I'm having a party, in which case
everyone will be too drunk to worry about sound quality).

Here's the history of my gear (I started well, I think):

mid-'80's

Nagaoka MP11 cart
Dual CS-505-1 deck
NAD 3020 amp
Key Coda 2 speakers.

A pretty classy setup for an 18 year old! I think I must have had very
good advice, in hindsight.

The NAD was stolen, and replaced with a used Technics SU-V303 amp. New
Class A, pretty chunky piece of kit (seems to be the same as the
SU-V505). I wonder if this is better or worse than the NAD? - never
got a chance to do a comparison, obviously. My feeling is that it
isn't superior. I might go back to NAD.

The Dual deck was replaced with a pair of Technics SL1200's, with
Stanton AL500 carts. I know the carts are a huge weak spot - I intend
to get another headshell, and put a decent cart in for normal use, and
keep the Stantons for parties. Which cart I wonder? Grado?

With the Technics decks, in came a nasty little DJ mixer, which I know
I need to bypass for normal use. It seems a Y splitter for the outputs
of one of the decks and the CD player, to give me a choice of going
through the mixer or direct into the amp is a reasonable way to go.

Somewhere along the way I picked up a CD player - a well reviewed
Sherwood. But as it's my feeling that most CD players are pretty well
the same, and it's quite new, I'm not so interested in replacing that
yet.

The Kef Coda 2's have been retired (you will see them on eBay an day
now) in favour of Kef Q15.2's.

Now:

{Sherwood CD
{Stanton AL500 cart Technics SL1200 decks
(nasty DJ mixer)
Technics SU-V303 amp
Kef Q15.2 speakers

Where next?