Audio Research VS60 amp Matched tubs


I have two new matched 6550 tubes and two old tubes on VS60 amp right now. Do I need to buy QUAD matched tubes or I can just buy other two matched tubes to replace old tubes?

Regards,
Eddy
eddy1
Thank you.  If I change from 6550 to all KT 120, is sound quality will be better than 6550?

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