Fingers are starting to hover dangerously close to *checkout* and *ship to UK*..... The Crack + HD650 might just be the first setup I'll use to start my adventures in tube rolling!
Apart from the SB upgrade, are there any other upgrades/tweaks/tubes that provide noticeable improvements?
The SB upgrade is the most essential. I would say second is the tubes. The Tung Sol 5998 is a no brainer...sexy looking too. There are other updates but this is where you want to spend an extra if you have it.
Guess I really have to go with the Crack when I want to move on from Vali. Btw, does anyone know good inexpensive cables for the 650's? Not expecting to hear any difference in SQ, but I find the stock cable unnecessarily long. Bumbed into some Toxic cables for ~80€, but I wonder if there's still some decent options for cheaper, preferably in Europe.
Best cables on the cheap are DIY. Chop the extra length off and reterminate with a nice plug of your choice. I did this very thing with my 650 cable and it's much more usable now. Beyond that, BTG Audio is very reasonable.