What are best power tubes for the ARC -D250 ?

I have the Audio Research D-250 Power Amplifier, which currently has the Phillips (made in USA) 6550 tubes installed. I decided to install a set of 6550 Svelana (which has about 50 hours playing time on them-they were purchased new a few months ago.) They sound very dull in comparison to the Phillips 6550, does not have the bass punch, nor midrange clarity nor the highs! Is this the performance of the much vaunted Svelana tubes? I did not realize how bad they sounded until I compare them with the Phillips 6550's. What is happening here? Are there other better 6550's tubes than the svelana? Please I need your advice