Perhaps somewhat back on topic, I'm not a billionaire, but I have enough money to buy the best (at least when it comes to headphones) but I don't have enough that I want to throw it away without thinking about it. But if I just went nuts, to buy the ultimate system, as of today, I would buy:
Sennheiser HD650's with Zu Mobius
SinglePower SDS, balanced configuration, plus whatever upgrades and tubes Mikhail recommended for the HD650's after I told him I wanted to minimize the Senn "veil" (maybe even ask Mikhail to build two, for dual mono)
Porta Cross - for those times I want x-feed (I love x-feed)
Source - I would keep my Chord Blu/Dac 64MK2. I love the dynamic, plus it offers me the greatest flexibility with great redbook audio, plus a good DAC for those few times I want to use my computer as transport. Frankly, I would consider ultimate hi-rez SACD or DVD-A or a turntable, but 99% of my music is on CD only.
But even with all this, my mid-level speaker-setup would easily trump.
Best regards,
-Jason