I have both cables Angerer with the hd600s and I've tested them a bit
I prefer the hd600 cable with the hd600 headphones me.... there is a small difference
dont make me try and explain it though
I think the hd650s cable sounds bit muddy or maybe its a mind thing .
just stay with the hd600 cable mate trust me, you wont be playing with cables when you've built that amp
And don't mod the amp right away
build it stock and get used to the sound wait a few weeks months if you can .(dont install the speedball 1st build.
Also use Cardas Quad Eutectic Solder and you could upgrade the rca connectors with the nice cardas ones.
http://www.hificollective.co.uk/catalog/cardas-ctfa-socket-pair-p-9629.html they fit no problem and good quality.
also dont change the volume pot till later after doing other mods I say
..change the ouput caps to film caps 1st mod.
(I use Mundorf Mcap MKP 100uf 250v there ok but haven't tried anthing else yet as Im happy with the sound but I'm thinking of trying some new ones as you do
but very happy at the moment
I tried expensive volume pot but like the bottlehead one more ...dont underestimate the stock volume pot
And start searching for that tungsol 5998 tube (chatham also make a 5998 tube but make sure its genuine tungsol spec with the dimples on the plates in side and try a mullard with it ....I use a mullard cv491 but very rare expensive but most mullards sound very good with the hd600s
where about in London are you?
Have fun