What 300mm 2.8 lenses play well with 2x teleconverters?

Tokina and Tamron both have discontinued models. Sigma and Canon have both discontinued and current lenses that could do 300mm 2.8. I can't afford the newest crop of Canons. I tried a used Sigma 120-300mm 2.8 EX HSM (non DG) with my 2x II extender but it was very disappointing at 600mm. On a tripod in a controlled environment it still couldn't begin to approach the IQ of my 200mm 2.8L and 2x II extender.
What has your experience been and with what lenses/teleconverters? Thanks!
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