“Get the clock,” they said.
“It’s a matter of time before you buy the clock,” they said.
They were definitely right in my case 
I’d been loving my Bartok (with the headphone amp) for a year or so, but giving the classifieds (and dealer demo lists) plenty of side-eye, knowing that adding a Clock was likely to make a big difference.
After plenty of deliberation I ended up getting a demo Vivaldi Clock instead of a Rossini Clock, and I’m really glad I did, especially because headphone listening and a sensible(ish) box count are important to me. The only person I found who’d compared the two master clocks on a Bartok was @nickghough (in this very thread), so I gave it a go myself.
Like all of the audio purchases I make, I went into it hoping I wouldn’t be able to tell the difference.
Turns out that the differences weren’t subtle. I agree with Nick. The Vivaldi Clock transformed the Bartok. (It was already the best DAC I’d heard.)
As @PAR has said before, it wasn’t any of the normal audiophile stuff (better bass, sparking treble, blacker background etc.), that I loved, it was that it sounded as if the people playing had been practising for longer. The music hung together better — and because it sounded like a more capable band/orchestra/rapper/electronic plip-plopper, I was more relaxed for each listening session. No tensing up for a less well-sorted bunch of musicians, if that makes sense.
My next comparisons were:
A) Bartok with Vivaldi Clock
B) Rossini with no clock
C) Bartok with Vivaldi Clock and SRS Perf 10 Reference Clock
D) Rossini with the Vivaldi Clock and SRS Perf 10 Reference Clock
No surprises that the Rossini with all the clocking was best. It should be. What a sound! (Hoping for Bartok 2.0 soon.)
What was a surprise was that I preferred the Bartok with all of the clocking to the Rossini without any external clocking. The Rossini without the Vivaldi Clock was amazing — of course it was — but it didn’t have the relaxed ease that I wrote about above, and that I can only assume comes from such wonderful clocking. That ease is addictive.
Onto another comparison, at a mate’s place:
A) His Rossini and Rossini Clock
B) His Rossini and my Vivaldi Clock (but no SRS Perf 10 — humping one large box two hours in the car was enough)
His colourful language suggested to me that he was now shopping for a Vivaldi Clock. It took all of 15 seconds for us to prefer the Vivaldi Clock.
Quite how two boxes that ensure 1s and 0s stick around for a consistent length of time can be so different, I don’t know. But I’m glad some people at a CB24 post code do.
I feel like an arse for getting someone else into shopping mode. Sorry, @magicbus. Maybe it’s just referred naughtiness — thanks for the Reference clock, @PaleRider 