Audiovector QR7 or B&W 702 S3 to Diablo 300?

Not impressed with Joseph Audio. I find his designs mediocre sounding and too expensive.

Prefer the B&W.

Neither.  At that price level I’d go with Joseph Audio Profiles hands down.  It offers a good amount of the Perspectives at less than half the price.  I’d also take the Usher Mini Two DMD over the other two, and those only cost $5300.  Of the two you mentioned I’d take the QR7 as I’m not a fan of the B&W 700 series —too sterile sounding to me.  FWIW, and best of luck.