In a previous lifetime I biamped. I replaced the integrated and the associated power amp with a NAit 5. Before getting to the Nait, I used a EL34 pushpull amp with passive pre-amp. My conclusion is that it's the pre-amp that's the limiting factor.
I'm convinced that biamping is not cost effective compared with having a better pre-amp. So in your case getting the 150 and using in biamp mode, if it were possible, would not give you much more than your Nait all by itself, because you're still using the Nait's pre-amp section.
If you want better with the Nait a Flatcap/HiCap is the better option. If the 150 is a step on the way to a 112/150 better to use the Nait as a stop gap pre-amp, and then sell it later
Peter
