Jelco SA-750D or Rega RB250

But which tonearm sounds more welly? Which one has more midbass bloom?

It doesn't work that way. I mean if you stick an AT440MLA on either tonearm, you are not going to get midbass bloom. 

As for the collar, yes the stock one does not cause severe problems and is fine for most people.  However, if you try to print out and use an arc protractor, you cannot trace the arc well because the arm pillar is not vertical.  May or may not be audible to you but what I say can be easily verified.

So you're telling me AT440MLA will have the same exact midbass on both arms? I find that hard to believe. 

Jelco SA-750D.  Period.  I can't think of any Stock Rega that is in the same league.  Haven't heard ALL the arms using Rega parts, but EVERY Rega-based arm has the same sonic signature. 

Too right bp.
Why anyone buys a TT that they know will need many bucks to "modify" is beyond me .