FXLQ, IBFX and FXSOL all use the same (shady) clearing bank, Forex Liquidity LLC. http://www.nfa.futures.org/basicnet/Details.aspx?entityid=0196740

FXSOL and IBFX use Forex Liquidity (FXLQ). Velocity 4x appears to be a white label broker for FXLQ but they won't admit to it directly. They use the same marketing materials regarding their connectivity though (claiming a massive pool of 3 banks or something silly that FXLQ spoonfeeds people). 3 banks is NOT the same as 3 retail brokers.
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FXDD uses a Switzerland based clearing house for liquidity, Tradition Group. They have the same address as Tradition Group's NY office. Here are the main links for research. Tradition Group with Corporate information. (Firefox has issues with this site)

Also please compare the address for this NFA Registration With this address:
Tradition (North America) Inc.
75 Park Place, 4th Floor
10007 New York
United States of America
Phone: 1 212 791 45 00
Fax: 1 212 791 34 41

From their website list of locations in New York.

I think Tradition Group is no small fish considering they invest in developing nations debt, Energy, Metals, Equities, etc and has dozens of offices around the world.

I did a lot of research on FXDD before becoming involved with them. Tom O'Reilly and I have also spoken a number of times, he seems pretty straightforward and he follows through in my experience.