I was hoping someone else would jump in with their opinion but I can of course give mine. It's an interesting circuit, the first stage has a huge low end, but the second stage filters some of that out, and the tonestack is voiced a little bit brighter than the typical Alembic tonestack I used in Particle Accelerator/Underground Accelerator. It has a little more grit than Particle Accelerator due to the tonestack being at the very end rather than in the middle, so I would describe it as having a tight, focused low-end that's not overly warm but not overly bright. It has a nice punch in the low-mids, and while it doesn't have as much low end on the output as PA/UA it still has a nice fatness to it.
A common option was to get the Particle Accelerator tonestack added into the London Underground (which effectively makes it an Underground Accelerator) so if you could get a pic of the circuit board I could tell you if that has been done. It's a best-of-both-worlds thing in my opinion, keeps the grit of the London Underground with the rumbly low-end of Particle Accelerator.
Obviously I'm completely biased, but that's my view on London Underground.