Adding subwoofer to Quad 12Ls

I have my audio system in a large room, 15' x 30'. The Quad 12Ls started in a smaller room but have been doing a fine job in this big space for awhile now. They are matched with a Plinius 8100 integrated, and I listen exclusively to vinyl.

Should I add a subwoofer to this system? If so, should I stick with a Quad? or would it better to just invest new speakers?

Thanks.
Joe