Impossible to say without knowing what speakers are being used. Depending mainly on how their impedance varies as a function of frequency, some speakers will not sound right with one type of amp or the other.
It is also impossible to say without knowing whether the B&K amp or the preamp section of the Sony a/v receiver you are using is a greater limitation on the sound quality you are presently achieving. Not being familiar with either model, I can't address that question. And the answer may depend on the impedance characteristics of the speakers, and how difficult they are to drive.
As a general philosophical matter, though, if the speakers can work well with a tube amp I agree with the comments by Michael and Charles, and my vote would be tube amp, ss preamp. If you research past threads on this question, however, you will find that opinion is divided, and there is no consensus.
Regards,
-- Al