Interchangeabe tonearms with RB300

Looking to upgrade an arm but prefer not to replace the arm board. My RB300 connects to the arm board using a 1 3/4" (approx) threaded connection and a nut that tightens from the bottom. Are there any other arms that would be an upgrade that use this same configuration?
The Continuum Copperhead is a drop in, as is the Graham Robin. The former would probably give you a good improvement in sound quality at the cost of a bit of added complexity.
Hadcock 228 Export
The VPI arm screws into the top plate with wood screws...not hard at all. It's not a "drop in", but is very easy to attach.
Origin Live makes several tonearms that may work.
I second Kbell's recommendation of Origin Live tonearms. They have a nice product selection from very reasonable to very expensive and are not only terrific sounding but a great value. I believe Jay at Audiorevelation carries the line.