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
I think the 6550's ARC currently stocks its big tube amps with are the Svetlana 6550C's. They are different from plain 6550's, supposed (of course) to sound better. They certainly sound great in my ARC VT100 Mk III.
I have the Svetlana 6650C's also. I thought that they sounded excellent in the ARC CL150's which I have. I tried them in the ARC D-250, until I compared them with the PHILLIPS 6550'S. For the sake of experimentation, try the NOS Phillips if you can find them, and let me know your verdict. You will be flabbergasted!
A friend has been using Tung Sol 6550 black plates for some time, although they are expensive.

Brian Walsh