Mcintosh C15 vs C36 or C34v

Hoping to get some advice here.

I recently bought a vintage  Mcintosh MC7150. It sounds really smooth and silky with vocals and brings out the soundstage with a tad of warmth but still quite detailed. However, somehow the bass does not have enough weight or slam though it is quite tight. This is in comparison with my previous MC7100 which the MC7150 replaced.

My preamp is Mcintosh C15. It serves me well but at upper registers it can  get slightly shrill. Am thinking would I be better of with a C36 or C34v preamp? What would be the difference in your opinion? Better smoothness, bass? 

My speakers are Living Voice Avatar II and it's quite sensitive at 94db at 6ohms

Happy to hear your views.