Dear Richard: You already solve the TT issue, one important step in the music/analog reproduction. Now you have to deal with a more important step: tonearm/cartridge combo, your money would will be here ( if you love and want the music ). Your 300B can't do anything for improve the sound that comes from your analog front end ( anything can't do it ), so you need the best quality signal from your analog front end: you have to put your money here.
Then, if you care about music, you have to choose a tonearm with a good design and a good execution design ( it does not matters if is unipivot, gimball,knife or anything else ) ( the Rega is not a good design. That's why the people try to do changes on it and that's why exist Origin Live ): Moerch, SME, Audiocraft,Pluto,Vector,Schroeder ( Topoxfordoc recomendation ), are really great tonearms and do a good match with Shelter 901.
Regards and always enjoy the music.
Raul.