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mike_mccue
...Is there a Wurli package that excels at the lower register?

Have you tried MrTramp2?
http://www.genuinesoundware.com/?a=showproduct&b=40
 

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Arturia has a Wurly thingy too. It is supposed to be good, but I don't know about the low register. Arturia Wurlitzer V.

There's a decent wurli sound in Session Instrument: Electric Piano. I'm not sure which upgrade included it - but I've used it and it sounds great.

Mr Tramp,, I have no issues running it in 64 bit. It is my most used VST. I have it on way to many songs :) I love it.
Only glitch is the tremolo control will not stay put. Work around is pushing the joystick up on my controller will set it. So it's kinda like an organ leslie. 
Low end growls perfectly. I've tried them all. And I thanks to who ever it was on this forum for telling about it.