Subwoofer for a Primaluna Dialog II System

i have a couple systems i use to drive Magnepan 1.7s. The first is a MC402 Amp paired up with a C46 Preamp (McIntosh). I am very satisfied with the sound of these with the speakers. I also have a Primaluna Dialog II i use when i want to listen to a Tubes. I am very happy with this as well. Only thing is the base is not as nice as the McIntosh delivers. Was thinking of adding a Sub. Looking at the Rel T7. Another thought would be to change out the Driver Tubes on the Primaluna. Currently the Stock Tubes are the KT88s it came with. Thinking of KT-120s. Not sure how much more low end punch this would deliver. Looking for ideas.