I do think it might add some of the 300B weight. How much 300B weight may be somewhat dependent upon the headphones you are using and how well your source works with the input impedance of the Mainline vs the higher input impedance of the BeePre.
I suspect that the 300Bs would show their stuff pretty well through the HD6XX and the Utopias. I have not listened to the TH900s, so I can't speak to those.
thanks folks. I'll think about it. I hope to be at the Seattle AudioFest this coming summer, assuming they have the show and the vaccine distribution has gone really well (BIG if's). Maybe I'll have an opportunity to listen then.